That’s because Navy has Marines playing for the team!
Semper Fi!
That's not too far from the truth. When I was in Air Defense Artillery Officer's Basic in '76, our flag football team had several Naval Academy grads from the class that followed ours. Most of them played on the Navy football team, and we were dominant.
So dominant that we were split into two teams, because the powers that be couldn't stand the thought of a bunch of butterbars winning the Post championship. Both teams gave it a run, though.