if you take a team and say they are the under dog in 6 games by an average of 7 points, the chances of them actually losing each of those games is actually probably not that high.

Just like when you do a parlay. The casino knows you will probably guess 2 out of 3 or 4 out of 5, but picking them all is not so likely, that is why the payout is much higher than betting straight up on one game.