A strength roll is a strength roll.Except where they explicitly line up with real world weights and measures. Though I agree, it's an abstract fantasy game. I'm good with that. If anything, it's too worried about being concrete and not abstract enough.
Raw strength score Hafthor Bjornson, by a mile. They're both trained in athletics, but Phelps clearly has expertise in swimming...or we just acknowledge he's a bad example because of his several genetic advantages that give him basically the perfect body for swimming, larger lung capacity than most, and his body produces less lactic acid, amongst other things.