This is the only exception. The issue is with weapon damage. Hits have native weapon damage attached. The 186 weapon damage is higher than 174 PvP (regardless of what bolster says). The reason you mix is because the EXP hit is sizeable, but by carrying EXP on the other slots, you overcome this. You still get an EXP nerf, but you get +100 or so damage on high level attacks. When I did the calc it was a 4% increase in attacks across the board...
This issue has been present since bolsters inception, and even though I explained to Tait why and how it was broke, they didn't comprehend that weapon damage bolster doesn't override native weapon damage on hilts. I have done any testing on this in awhile, but yea. I wouldn't bother getting a pvp weapon until the very last item.