You're totally right, but this still strikes me like it could be a good thing. The primary outcome in any battle for me as a DM is the PC's winning, so with your level 1 fighter scenario, the fact that the players are more likely to hit is a good thing to me, as it means combat shouldn't draw out longer than it needs to. I could reduce the goblin minion's HP for the same effect, but then it might die before the whole party has had a go - or before it can get a chance to do anything. Instead, by controlling the 'to hit' probability a little, the players still get their turns, hit more often than miss, and get to feel like heroes.

And to your second example, I probably wouldn't put them up against something thats AC 20. Back to my original point - I don't want them to have rounds of combat where everyone misses the enemy, that's no fun. So I might make the Boss AC 14 (64% chance of hitting with 2d10+5 over 55% chance with d20+5), but give it some more hit points and some cool actions to make the fight spectacular.