of the possible outcomes, you are setting the book to maximise your take.
So you might start with estimates of the probabilities, but you are going to
adjust these as you go to reflect the way the betting is going.
(If we are talking soccer here, there are not really enough results in a
season, and the statistics are not stationary from season to season,
to develop a good model, at least not much better than gut feeling)
Oh, and if we really knew the answer to your question we would not be
telling, instead we would be taking the bookies to the cleaners every week.