this guy I knew, went to marines basic or army, whichever one does pullups. Anyway, he was one of them freaks that was always capable of doing crap load of pullups. He weighed something like 153lbs and 5'9 so he was a lean guy. Anyway before he left for training he could do 26 pullups nonstop, when he came back he could only do 18, boot camp screwed him over. Said he was sick the whole time, went to the hospital at one point, dropped down to 140lbs, worst time of his life. He now is out of the military and he has a lot of resentment towards the entire military now. He actually got out a year early but I dont know why, he says medical leave or something but who knows.
Anyway, pushups as everybody has said arent a very good way of gauging strength, pullups are definetely quite better.