Actually, from my understanding DT works by subtracting that amount from the damage. (Along with an automatic 20%, but thats not the issue here) So that makes it Damage Reduction, or DR.
Fallout 3's DR was Damage Resistance, which was only a percentage reduction from the damage.
This was the reason for making this because apparently Obsidian doesn't understand what threshold means, as this is how it works in Fallout 1 and 2 as well.
If by 'this is how it works in Fallout 1 and 2' you mean the Threshold value works like you say it should work, you're wrong. DT in Fallout 1 and 2 worked exactly like it does in New Vegas if you set the leak-through amount to 0.
Also, 'threshold' is still correct - the threshold being when damage begins to increase past 20% of weapon damage.