Well, if you want to break it down like this, A-Rod scored 2 runs and drove in 2. One of each was the home run, so he scored or drove in 3 runs. There's your margin of victory.
But there's more than that. In a baseball game a lot of what happens is interconnected. If you hypothetically change one thing a bunch of other things can change too. For example, managers make certain decisions based on the game score and situation. Farrell might use a different pitcher in a tie game than he would when we're behind by 2 runs. That type of thing.