My best guess is that Sario thought he knew who the first mason was, and was confident that he’d have a vote.
Look at how people’s “probable town” lists have played out up as a result of that vote. We have some people who are convinced that those who ended the day with a Sario vote is obviously town. If things had gone the other way, then Sario would be on those lists instead.
If Sangfroid is a wolf, Sario’s vote may have been a calculated risk that backfired.
Or it may be that he was never paying attention to the game at all and had no real plan in place.