Favorite positions are by default determined by using chance value. Chance value is calculated by using back and lay price e.g. 1 / ((back price + lay price) / 2.0)
back price 2.0
lay price 2.04
chance value = 1 / ((2.0 + 2.04) / 2) = 1 / 2.02 = 0.495 and that is 49.5% chance value
First favorite has highest chance value and outsider has lowest value.