I usually type my message, then reread it and optimize/shorten it if possible before submitting. Sometimes I even discard the message entirely.

That would give you an average message length that's shorter than what you typed. In the case that it was longer, it turned out those users were copying and pasting the content, then maybe editing it a bit before sending.

Oh, I got it now, so there were more characters than they typed.

Yes! A clever trick to catch out spammers.