UngeheuerLich
Legend
I think maybe you're misunderstanding the math here. (Or I'm misunderstanding your point.) The 20% is average. On any given turn you can't "count" on anything.
No. I perfectly understand the math. You do miss the point. Mechanically the best choice is correct, but since we are playing a roleplaying game, there are feats like actor which might be a lot more impactful. A +1 bonus here or there makes no (big) difference for the overall outcome of a single battle. It might increase your chance of survival over the course of an adventure, as it might turn miss into a hit at the perfect time, or you might survive with 1 hp or you could keep up concentration if you had +1 more con bonus. The +1 will save or perception check might also help.
And if you had the choice in point buy to have either 15 Str and 8 wis or 13 Str and 13 wis, you would trade a +1 bonus for a +2 bonus. So may be, the opportunity cost to get that mechanically best extra +1 modifier might be too high.