Opsec.
He was trying to throw people off his scent.
Satoshi was a pseudonym -- this person does not exist under this name. He used disposable emails. He paid for the domain with an anonymous registration agent. Presumably he only connected with tor.
He didn't want to be found, and he went to great lengths to ensure that.
So, 'z' versus 's' was a way to throw people off his scent.
The way to analyse this would be to graph is 's' versus 'z' over time. Did he use 'z' for the first half of his posts, or the last half? Are they consistently mixed at all times?
And no, I don't believe that Satoshi was a team of people. Single, clever individuals do great things all the time by finding combinations of old ideas into new.
Very few people in the world could keep a secret like this for so long if they were a team.
I don't believe the OPSEC theory. I don't believe international English and GB English was used to throw people off his scent. I'm believing Satoshi Satoshi Nakamoto was a pseudonym used by the creators of bitcoin.
The more that I read about Satoshi, the more I feel as though he/she was more than one individual.
My own problem with this presumption is that this relies on all of those individuals having perfect OPsec.
If it was one person, he likely had the most bizarre sleep schedule of all time. His posting hours were never consistent.
If Satoshi was one person it's hard to look past Hal Finney. He died in 2014 but he's standing out the most. If Satoshi was a group he was probably involved in it.