The first thing that you’re doing is to assign behavior to alignment. Let’s say we have a person, where when he’s town he always does action X, and when he’s scum he always does action Y. You know this because of the many games in the past where this pattern has held. He now performs action X. For that reason, I would consider that person town.
And for what it's worth, this is why I don't like talking about in progress reads. Because suppose you have in your mind a set of actions X that a town-aligned player would take, and you see them not doing that. You call them out in thread, "Why aren't you doing X?"
Then, both a town-aligned player (who had some excuse for not doing X before or maybe they were going to do X anyway) AND the scum-aligned player would want to immediately do X. They both have motivation to do it. So now you've not only ruined your read on that player for this game, but you've ruined it for all future games you're playing with that person. X no longer splits alignment.