Biological fact: people are so varied that any 'average' is useless for measuring individuals.
"Men have more upper body strength" okay, but in how large of a sample size do we need before that evens out to that fact?
Not many, to be honest. I have seen studies with as few as 8 individuals of each sex and studies with hundreds. Both show the same fact: when it comes to muscular power, males (as a collective average) are stronger than females. In one study, males had an additional average of 26 lbs. of muscle than females. It is a slightly higher ratio in upper-body compared to lower-body, but males have more in both areas.
However, many of those studies show females have better muscle longevity, a more robust immune system, are better at coping with trauma, etc. Since those aspect (for our game) are more represented by exhaustion levels, we gave females one extra free level; so they need to have two levels before they begin to feel the effects.
In practical game terms, this means a male fighter with three levels of exhaustion will have disadvantage on attack rolls and saving throws, while a female fighter would not have that penalty yet.
I should add: at one point we did do a male = STR +1, female = DEX, CON, or WIS +1 (player's choice). However, not wanting PCs of either sex to have a chance of less bonuses, we moved to the current system-- which incidentally we like much more as it represents the factors we wanted to incorporate.