It's basically because of the two different branches real physical tests
ie:
APFT (Army)
2-min pushups
2-min sit-ups
2 mile run
PFT (USMC)
Max pullups
2-min sit-ups
3 mile run
As for why each branch is different, you got me. You'd think branches with similar missions, or even the entire military, would have one single standardized physical test.