Think I understand what you're getting at. Who's going to bet that the Cubs get more than 89.50 wins? Cubs fans. Who's going to bet that they get less than that? Oh, say, Cards fans. And Pirates fans. And Brewers fans. And anyone else who likes another team better. So if by putting the over/under at 89.5 they get the same number of people who bet in favor of the Cubs as who bet against they Cubs I'm guessing they achieve their mark. If that logic holds, that explains why nobody is above 90 wins even though most predict the Cubs and others will be considerably higher than that.
I don't really know much (hardly anything) about Vegas odds-making, but if that's your point, I get it. Then again, I'm about half drunk right now, so the logic portion of my brain probably checked out a while ago...