You'll find that people that have english as their first language tend to make that kind of mistakes more often than people who have it as a second language. In my case, for example, my first language is spanish, where everything sounds as it's written. When I see mistakes like writing "manor" instead of "manner", or "should of" instead of "should have" I can't help but think that I would never make that kind of mistake, because of the way people learn english as a second language in comparison to a first language.