Mini-FAQ on Win Value
The most frequently asked question on Win Value so far is a variation of this: "Player hits a go-ahead home run in the 6th. Later his team scores 6 more runs in the ninth, and wins by 7. Do these later runs affect the Win Value for the home run?"
The answer is no. Win Value is determined at the time of the event, and is not affected at all by subsequent events. When that player hit that go-ahead home run, if the game played out according to the averages, the home run would have a certain value in that context. That is Win Value. The fact that the game turned out differently than the average shouldn't change the value of the home run. That is the only way to provide a reliable benchmark against which different events can be measured and compared.
For more detail, please see this post