Opsec.

He was trying 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?

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.