When playing an opponent and winning, are the points taken from the opponent's Elo at the beginning of the game or end of the game? For example, I may play someone who begins at 1900 and is at 1600 when the game ends as they have played other players at the same time.
My guess is that the ratings used for the calculation are the ratings you and your opponent had at the beginning of your game. The change of rating is applied on your ratings after your game is done. For example
Scenario: when the game starts, your rating is 2000 and the opponent's is 1900. When the game finishes, your rating is 2150 (due to other finished games) while your opponent's is 2100. Then, under my assumption above, the following happens (if you won the game)
Your New Rating = 2150 + 2000_wins_against_1900
Opponent New Rating = 2100 - 1900_loses_against_2000