People are assuming that if he didn't lose his leg that he would still be strong. There is always the possibility that because he lived through losing his leg, he changed his whole outlook on life and became more dedicated to improving himself leading to the strength he has today. Without the motivation of losing his leg he may not have put in the work required to improve.
Also there's the small matter that if he didn't lose his leg, he never would have found and raised Angie so his luck stat is fine; It just works in extremes.