This has happened to me with numerous other occasions and it seems like something maybe wasn't coded right? Another thing I have noticed is with the Anti-Material Gun. When I go into my pipboy and I have some other weapon equipped, if I hover the cursor next to the Anti-Mat Gun it's DAM says something along the lines of 167 or something in that area. So I equip the gun and all of a sudden the DAM goes down to 111-112 <---somewhere around there) again I meet the skill and STR requirements.
Also with AP bullets it seems like the bonus DAM is calculated after the initial hit? Instead of being calculated into the shot itself if you understand what I am saying. Like if the shot would originally be absorbed it still gets absorbed instead of actually breaking through the armor.
Maybe I am just not understanding the way the system works, but from what the game describes on the info-loading screens and also the instruction book something isn't working right IMO.