They work really, really hard. If they say they don't, they're lying (either about not working hard, or about how great their grades are).
When I was in law school, two friends and I came in one Monday morning, all saying we had "blown off" studying for the entire weekend. One of them ended up being the top student in the class, and the other friend didn't graduate. I did well enough to be on the Dean's list all three years, but I was not in the top 10% of students.
Further discussion revealed that when I said I had blown off studying for the weekend, it meant that I went out Friday and Saturday nights, and didn't do any work on Saturday at all, or on Sunday morning, but Sunday afternoon and Sunday evening I read my assignments and prepared for Monday's classes. The friend who didn't graduate, he meant he literally did not pick up a book all weekend and was not prepared for Monday's classes. To the eventual top student in the class, she meant that she studied Friday night, all day Saturday, went out to dinner with her husband on Saturday night, and studied all day Sunday and Sunday evening.