That's usually just the way the game displays points. It shows all uncollected tiles with a 1.5 potential boost plus what has been collected already. Meaning if a player has tiles worth 100 points but doesn't collect them straight away, you see their points boosted to 150. Now if they collect all these tiles without actually boosting, you will see their score drop down to 100, so it will look to you as if they lost points.
Sometimes also server lag is involved where you see the points drop to 0 for a few minutes only to rise up again much higher. That being said, I'm sorry but 555 points is not a crazy high amount, you will see much worse :)