EDIT to clarify: ... what is it? what are its stats? Anybody have any info on this?

There're plenty of items you cannot access. They're part of the 'hidden inventory' of a NPCs

As for why, these are considered 'one of a kind' items (think about Tsun armor) without a specific different mesh for females (or vice versa) and that doesn't work with all different combination of apparel/clothes.

If you get the ID code you can equip them on your character via console. Chances are, however, they will glitch or they will not be shown in the inventory at all

right i kinda knew that, but does anyone have any info on it's stats? is it heavy armor? how does it compare to steel, dwarven etc? there is no info about it on UESP

If you can't remove it with Perfect Touch perk from Pickpocket or remove it if you kill him then it is simply his clothing (or may as well be)...

um, stats? heavy or light armor?

Just a guess... 0 armor 0 weight and 0 value. That's it for most non-removable clothes