Exactly why I clicked the comment link. The differences in behavior could also be caused by a difference in psychology between students. e.g. the elite group love what they do and the other group don't. Which could indicate why the other group spread their practice throughout the day - because they're procrastinating all day long.
I love what I do (my hobby - swing dancing). I do it for 3-5 hours a day (after work). I would improve much faster if I danced less, spent most of the time doing hard work (cleaning my moves, learning more moves, more cardio, ...) and rested more.
Right now I'm always on a tired state, and improving quite well, but finding myself lacking in a lot of ways. When I take a break my improvement is greatly enhanced.
What it may come down to is simply strength of self-discipline. What separates the achievers from the middle-of-the-roaders is self-control. The classic "marshmallow experiment"[1] would seem to bear this out.
[1]A famous longitudinal, social-psychology experiment in which children were offered a marshmallow (or a cookie, depending on which study), and told that if they didn't eat it, they could receive a second marshmallow in five minutes. Then they were left alone for five minutes. Most children ate the marshmallow. The ones who didn't -- who waited it out and resisted temptation -- were also the ones who grew up to be more successful, earn higher incomes, and perform higher on tests of intelligence.