Hm. That's an interesting question. It's what I've heard from many people ever since the game came out practically...I'd also read it was some kind of effect/script bug, rather than native. But I've never actually tested it for myself. (edit)It's also possible if was a bug of some kind, that it was fixed/altered in the last patch.
So here's what I did - I GECK'd a Grunt Armor to have 500DT. I don't know what the 'max DT limit' for DT the GECK has, so maybe that's more than the limit and only some of that truly registers with the game, I have no clue...but anyway, I then ran Boone over to Sloan and had him engage 1 deathclaw with a combat knife (so Boone couldn't do much dmg to dc). Using debug text, here's the result: (it's long so putting in a spoiler tag)
Spoiler
Boone hits Deathclaw for 37.3/500.0 Health damage!
Deathclaw hits Boone for 20.0/300.0 Health damage!
Deathclaw selects LEFT power attack!
Boone selects RIGHT power attack!
Deathclaw hits Boone for 20.0/290.0 Health damage!
Deathclaw hits Boone for 20.0/280.0 Health damage!
Boone selects RIGHT power attack!
Deathclaw selects RIGHT power attack!
Deathclaw selects FORWARD power attack!
Boone hits Deathclaw for 22.3/481.3 Health damage!
Deathclaw hits Boone for 40.0/270.0 Health damage!
Deathclaw selects FORWARD power attack!
Boone hits Deathclaw for 37.3/470.2 Health damage!
Deathclaw hits Boone for 20.0/250.0 Health damage!
Deathclaw hits Boone for 40.0/240.0 Health damage!
Deathclaw selects FORWARD power attack!
Deathclaw hits Boone for 20.0/220.0 Health damage!
Boone selects BACKWARD power attack!
Deathclaw hits Boone for 20.0/210.0 Health damage!
Deathclaw selects RIGHT power attack!
Deathclaw hits Boone for 20.0/200.0 Health damage!
Deathclaw hits Boone for 20.0/190.0 Health damage!
Deathclaw selects FORWARD power attack!
Deathclaw hits Boone for 20.0/180.0 Health damage!
Boone hits Deathclaw for 37.3/451.5 Health damage!
Boone hits Deathclaw for 22.3/432.9 Health damage!
Deathclaw hits Boone for 20.0/300.0 Health damage!
Deathclaw selects LEFT power attack!
Boone selects RIGHT power attack!
Deathclaw hits Boone for 20.0/290.0 Health damage!
Deathclaw hits Boone for 20.0/280.0 Health damage!
Boone selects RIGHT power attack!
Deathclaw selects LEFT power attack!
Deathclaw selects FORWARD power attack!
Deathclaw hits Boone for 20.0/270.0 Health damage!
Deathclaw selects LEFT power attack!
Boone selects LEFT power attack!
Deathclaw hits Boone for 20.0/260.0 Health damage!
Deathclaw selects RIGHT power attack!
Boone selects NORMAL power attack!
Deathclaw hits Boone for 20.0/250.0 Health damage!
Deathclaw hits Boone for 20.0/240.0 Health damage!
Boone selects RIGHT power attack!
Deathclaw selects LEFT power attack!
Boone selects BACKWARD power attack!
Deathclaw hits Boone for 20.0/230.0 Health damage!
Deathclaw hits Boone for 20.0/220.0 Health damage!
Deathclaw hits Boone for 20.0/210.0 Health damage!
Boone selects NORMAL power attack!
Deathclaw hits Boone for 40.0/200.0 Health damage!
Boone selects LEFT power attack!
Deathclaw hits Boone for 20.0/180.0 Health damage!
Deathclaw has run out of room to dodge.
Deathclaw hits Boone for 20.0/170.0 Health damage!
Boone selects NORMAL power attack!
Deathclaw hits Boone for 20.0/160.0 Health damage!
Deathclaw has run out of room to dodge.
Boone selects NORMAL power attack!
Deathclaw hits Boone for 20.0/150.0 Health damage!
Deathclaw has run out of room to dodge.
Deathclaw has run out of room to dodge.
Deathclaw selects RIGHT power attack!
Boone selects RIGHT power attack!
Deathclaw hits Boone for 20.0/300.0 Health damage!
Deathclaw has run out of room to dodge.
Deathclaw hits Boone for 20.0/290.0 Health damage!
Deathclaw has run out of room to dodge.
Deathclaw hits Boone for 20.0/280.0 Health damage!
Deathclaw selects LEFT power attack!
Deathclaw has run out of room to dodge.
Deathclaw has run out of room to dodge.
Boone hits Deathclaw for 22.3/421.8 Health damage!
Deathclaw hits Boone for 20.0/270.0 Health damage!
Deathclaw hits Boone for 20.0/260.0 Health damage!
Deathclaw selects RIGHT power attack!
Deathclaw hits Boone for 20.0/250.0 Health damage!
Boone selects LEFT power attack!
Deathclaw has run out of room to dodge.
Deathclaw hits Boone for 20.0/240.0 Health damage!
Deathclaw has run out of room to dodge.
Deathclaw has run out of room to dodge.
Deathclaw has run out of room to dodge.
Deathclaw has run out of room to dodge.
Deathclaw has run out of room to dodge.
Deathclaw has run out of room to dodge.
Deathclaw hits Boone for 20.0/230.0 Health damage!
Boone selects BACKWARD power attack!
Deathclaw has run out of room to dodge.
Deathclaw has run out of room to dodge.
Deathclaw has run out of room to dodge.
Deathclaw hits Boone for 20.0/220.0 Health damage!
Deathclaw has run out of room to dodge.
Deathclaw has run out of room to dodge.
Deathclaw has run out of room to dodge.
Deathclaw hits Boone for 20.0/210.0 Health damage!
Deathclaw selects FORWARD power attack!
Boone selects LEFT power attack!
Deathclaw has run out of room to dodge.
Deathclaw hits Boone for 20.0/200.0 Health damage!
Deathclaw has run out of room to dodge.
Deathclaw hits Boone for 20.0/190.0 Health damage!
Boone selects BACKWARD power attack!
Deathclaw has run out of room to dodge.
Deathclaw has run out of room to dodge.
Deathclaw hits Boone for 20.0/180.0 Health damage!
Boone selects NORMAL power attack!
Deathclaw hits Boone for 20.0/170.0 Health damage!
Boone blocks 1220% of Deathclaw's blow!
Deathclaw hits Boone for 40.0/160.0 Health damage!
Boone selects NORMAL power attack!
Deathclaw hits Boone for 20.0/140.0 Health damage!
Deathclaw selects LEFT power attack!
Deathclaw hits Boone for 20.0/130.0 Health damage!
Boone hits Deathclaw for 64.1/410.6 Health damage!
Deathclaw hits Boone for 20.0/120.0 Health damage!
Boone hits Deathclaw for 144.1/378.6 Health damage!
Deathclaw selects RIGHT power attack!
Boone hits Deathclaw for 144.1/306.5 Health damage!
Boone hits Deathclaw for 64.1/234.5 Health damage!
Boone hits Deathclaw for 64.1/202.4 Health damage!
Deathclaw selects LEFT power attack!
Deathclaw hits Boone for 20.0/110.0 Health damage!
Deathclaw hits Boone for 20.0/300.0 Health damage!
Boone hits Deathclaw for 144.1/170.4 Health damage!
Boone hits Deathclaw for 64.1/98.3 Health damage!
Deathclaw selects RIGHT power attack!
Boone hits Deathclaw for 144.1/66.2 Health damage!
Deathclaw stops combat.
Deathclaw is dead!
Boone stops combat.
I absolutely do not know the formula % stuff on DT vs. dmg etc. but I'd say it was doing something. Boone normally would go down in 2-3 deathclaw swipes (with a more normal 17-20DT) but he was obviously able to take a lot more than that (I did heal him a few times before finally letting him use his Boomstick rifle to finish the dc off).
P.S. - OT side note: For kicks I fast-traveled to one of the places where viper bandits spawn in crazy numbers almost on top of the FT point (thanks to the IWS mod I use) and despite my 500DT and using some stimpacks, I died in about 20 seconds because they'd still do 2.5-5dmg every hit +grenades (and I still had Boone on melee w/his knife). That was funny. I'm easily amused as always.