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Simply because, some people have a very good brain. They were born to study, one time read it, they can remember for quite a long time. Maybe they good at organize their time-table: gaming for maximum 1.5-2 hours, exercise well, out-going, talk to people, read books, think wisely, etc...
Like me, I'm not very out-going and usually gaming for more than 3 hours (lol), so I'm suck at History, Geography, Biology, anything that involve learn-it-by-heart. But on the other side, since most of the games I play are physics games, which is require you to think and calculate, I'm good at Math, Science and Physics.