This has happened to me a few times. Your opponent was most likely playing multiple games at once. One of his other games ended before yours. As a result his rating was different at the end of the game than in the beginning which may have slightly altered the point allocation.
error in chess rating calculation??
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I'd like to show one strange thing which happened today, when I was playing my first game. If I'd won I should have won 299 points, then I actually won, but I was awarded just 296. (I have included the image) It doesn't make sense to me, because if the formula already knew the number of points I would have got for winning, then why it changed after I really won?