It can be obtained by just increasing it to this value during character creation.
Like, you know, any other value in any other stat between 4 and 20.
So I really fail to see why 18 is not “normal” from a game-play perspective. Your argument is based solely on some piece of lore stating this should be “superhuman”, but as a matter of fact it’s a possible starting value just like any other possible starting value.
Anyway, I don’t really want to do into details for the rational, the problem is (and has always been) that 35 base damage on a bow is ridiculously unbalanced (overpowered) with the rest of the game.
P.S. The game is not balanced around pure realism and never have been. Don’t give me this “X lbs draw should give Y damage” in a game where you can get shot with 9mm into every limb, torso and head, and then be perfectly healthy again next morning.