I think that you are making a mistake if you consider to/too and their/they're/there as misspellings. Those are mistakes of usage which causes an apparent misspelling. As far as non-natives being better spellers than natives, I think you are looking at the BEST of the non-natives versus the AVERAGE of the natives. I think that would bring about the results you claim nearly every time with nearly any language. Look at the average non-native's spelling and you will find a different picture.