[StavrosK]

100% correct. Also, non-native speakers tend to make grammar errors, use idioms wrongly, etc. They (we) don't tend to mistake "to/too", "there/their/they're" and "your/you're", because second languages are usually acquired through the written word.

[vidarh]

Well, I do sometimes make the mistakes you give as examples, but only as I get very tired, when I curiously start to mix up all kinds of homophones and near-homophones with no apparent connection (particularly curious to me since, as you suggested, I picked up English primarily through writing; it's not like I have a habit of sounding out words. It took many years from I started reading and writing English until I ever used it in a spoken conversation)

For the same reason I can't ever see me using "should of" - it sounds too wrong.

It is rarer for me to make the mistakes you list than mixing up completely unrelated homophones, though, as they're definitively ones I'm extra aware of.