That, and the fact that this site is mostly used by Americans, therefore you will naturally see more correctly AND incorrectly spelled content from them. I can't say much about the ratio between them, but the sheer numbers affect the ratio between US and non-US users (yes, even including other native English speakers worldwide, like UK/AU).

But while it's anecdotal, people I know are just as bad/good at spelling in a foreign language as they are in their own, regardless of the way they learned either of those.