I will give an example. For example, we have an equal match of conditional Virtus.Pro and EG, one bookmaker gives a factor of 2 to win the first, another bookmaker gives a victory of 2.1 to 2.1 on bets dota 2. As a result, putting the same amount on two teams in different offices, we will be in the black with any outcome of the match. Such cases are quite rare, but there are. To calculate more complex ratios, you can use the calculator forks (easily searched in Google).