In tennis, despite the fact that you only have two outcome, bookmakers tweak odds in way that the house margin is baked in over two options instead of three.
I wonder if there's a statistic that shows what's the winning result for the bookmaker over many games. I'm not surprised if they change things so they won't get screwed by odds while still making sure that the game looks interesting for the players. I'm not familiar with other sports, so I can't tell just by seeing things at first glance.
I think that there is no statistics that shows winning results for the bookmakers because it's an inside information for every bookmaker and bookmaker service provider. But I can tell you this, bookmakers always build in a margin to ensure long-term profit. When there is a fight between two person, the total odd should logically be 4.0 (2.0 for each) but bookmakers make it to look like 1.96 for each (3.92 totally), which means that 0.08 is in their favor. In other words, for example, 8% profit margin means that for every $100, they keep $8. This is how they remain profitable even when individual bettors win bets but you also have to keep in mind that bookmakers constantly change odds to keep things in their favor to minimize risks and increase chance of gaining a profit.