He/they probably mixed it up so people can't easily figure out nationality.

With that being said, I honestly don't get the people's obsessions to figure out who is behind Satoshi Nakamoto name. Sure, all of us spend some time thinking who could that be, but I dont think that it could bring anything good to bitcoin, and I think that's exactly one of the many bitcoin's strengths.

do you have any other example where Satoshi uses "s" instead of "z" because this is the only single example you posted with an "s" and it looks like change-log which might have been written by someone else (like Garzik) and copied here.
even still, one example is hardly proof because it may have been autocorrect tool setting one variation over the other.

Endless possibilities here. How can we predict someone's nationality entirely from their typing habits alone? What if he was from GB English background but just used the words alternatively and he had habit of doing so? We can't say anything for sure how anyone type the stuff. In extreme cases what if he had his assistant behind the screen answering others on behalf of him? What if he had dialect behaviour where he used to switch between two different English modes, or may be as OP said having mixed backgrounds?

This study can go anywhere but not to the point proving whether he was X English or Y English country person.