The tally tool has a dedicated
thread if you wish to report a bug (accessible from the options menu via a button).
My guess would be that you might be missing threadmarks for some updates, and the tool's default is to count from the latest threadmark until the end of thread.
If the quest in question is the one in your sig, the latest update is not threadmarked, making the tool count from the one before it. It counts last update's votes for those who hadn't voted yet, and new votes for those who did.
I didn't notice it failing to pick up a vote, though. Mind describing the situation in the thread above?