At the start of the year you look at the probabilities and run the numbers.  You don't give BYU a win for a 80% probability game.  You give them .8 of a win.  When you add the probabilities, you end up with BYU getting 8.807 wins and Utah getting 4.647 wins.

Once the first game is finished, you change the probability to a 1 or a 0 depending on if they win or lose and recalculate.