People of the same size with the same training do not end up with the same strength; look at the scoresheet for a powerlifting meet. A 135-pound man is stronger than a 135-pound woman who trains exactly the same way. Hormonally, men are set up to have a higher percentage of muscle mass. I don't know enough biology to describe it in more concrete detail, though.

Edit: I assume everyone knows enough stats to understand that this does not mean a female athlete can't outperform most men. I'm also not saying that women shouldn't challenge themselves athletically. I like the general thesis of "The Frailty Myth" and I think women could be better off training a lot harder than they generally do at present, and that a certain amount of female physical weakness is self-imposed. But there's also a biological difference.