English is my second language. I'd say it's due to the fact that when you learn a language in school, by listening AND writing, it'll stick in your head visually. I never mistake its/it's, they're/their etc., something I can frequently see native speakers do on the Web in their comments.
I also was told that non-native speakers can better distinguish between 'well' and 'good', and the usual suspects 'you', 'your', 'you're'. The latter is so basic that I am bewildered when native speakers cannot understand the difference.