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.