Maybe I'm just faster; my own experience is that I can push to 50 hours in a week, maybe 60, but then the following week I get about 20 hours of productivity in before burning out, so 60 isn't sustainable, at least for me. And those last 20 hours in a 60 hour week are really only good for brainless tasks.
God forbid I do any truly complex code in that time, or my net productivity will actually go negative, as I'll need to spend more time later fixing what I did as a zombie.
Of course spending time alone does not mean you'll be productive, but assuming equal intelligence, someone who is able to sustain concentration over a longer period of time will accomplish more than someone who concentrates for a lesser amount of time.