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Something like Football Manager works as a genuine agent-based simulation of soccer, but there's also a quick simulation which is probably just based on statistical models.

It depends on how you scale the strength ratings, a one point advantage in strength could mean a .75 win probability or a .51 win probability. Here is how they handle it in the Elo rating system:

http://en.wikipedia.org/wiki/Elo_rating_system#Mathematical_...

Modern sports games usually need the same stats from a simulated game as they would get from a played game so simulating is often just running a stripped down version of a full game.