First, to see what a game is worth in rating points, Thread 80000
Second, be aware that it's your rating status when the game finishes which makes the calculation.
The usual reason for what appears to be an error in the calculation is that you have had several games finish fairly close together, and your rating has been recalculated to a point where you didn't think you were.
So I suggest you go through all the games that have finished at any time in the 24 hours before this one, line them up in a row and look at the numbers. Your own rating graph can help with this.