A couple of points to remember:
Firstly, the ME is pretty sophisticated, so it could be the case that in-game your opponents are adjusting their defence to cover your SG more tightly than your PG.
Secondly, you can't know how good the guy covering him is.
Thirdly, the way games like this work means that players are regularly afflicted by streaks, or so it seems. Chances are, in a few games your SG could be burning hot.
Anyway, just to compare how unpredictable the situation is, my worst outside shooter (mediocre JS, inept jump range) is going along at a remarkable 45% from deep (5-11) and by best outside shooter (strong JS, average JR) is going along at 35% (18-52).