While the bookies are trying to equalize the bets, what really matters is the actual outcome. Shark take advantage of this. Thats why lines can move right before game time. If a bookie tries to equalize bets but then a large shark bidder recognizes that the bookie overestimated a team’s over/under, he can come in with a huge bet at the end and take it to the casino. They obviously don’t want that.

People have also looked at historical finishes of over/under bets and the bookies are incredibly accurate. The NFL is far more predictable than college football (which makes sense), but they are still very very accurate. They have to be.