That is confusing.
Viewing post in The Survival of Sarah Rose comments
I have to assume it's a simple coding error/oversight. My guess is when you talk to each person and get them on your side (or they're already on your side), a counter is increased by one. When you get to the vote, the game just checks that counter and uses that number as the vote count, when it should be adding one to that number to account for Sarah's vote.