How do Sports cappers's get their odds?

Here is my question, We have so many sports out there that people are able to gamble on. Granted, for some reason, it's not legal in the United States but where there's a will, there's a way. There are various sites out there that will still allow you to gamble using off-shore venues. But that's not the point, more just informational value. My question is, is there anyone out there that knows how spreads and over/unders are predicted? What are the methods used to pick for each sport?

I myself, love sports, and I love numbers. I put togeather different scenerios using past stats, player abilities and team records to tell me who is going to win a game and by how much that team should score. Once that's determined I can tell what the over/under is going to be. Does it work? Well of course not. Not always, and that's what bugs the crap out of me. When you have teams like Chicago Bulls who are tearing it up this season yet they lose to a team like Boston Celtics who are a nobody this year, How does that happen? According to my stats, Chicago should have been a favorite in the game by 10 points, but the sports cappers put them only at a 1 point favorite and the end result is they lost completely.

So, looking at this, tells me that the sports cappers know something more than what past stats and results tell us ordinary people. Is there anyone else out there that are like me? Do you do stats, self predictions on games and results? If so I'd love to hear your methods. Do you find them working or are they 50/50?