Because the Fitbit estimate of what it thought you would keep burning past the last point you looked at it was wrong.
When you synced the next morning, that corrected the prior time to midnight. Perhaps you went to be 1 hr early, and normally were more active. There's a mis-predict.
---------------------------------------------------------------------------------
Help the next searcher of answers, mark a reply as Solved if it was, or a thumbs up if it was a good idea too.
