A Lot of what Bookies do to cover their odds isn't directly related to the event itself, but more to balance out their risk on what people are betting on.
For example, If they took a bet on Team A vs Team B, and Team A was obviously much stronger, most people would want to bet on A, with very few betting on B.
Based on that, they will adjust the contest either by giving the game a spread (For example, You only win the bet if Team A wins by at least 10 points) or giving higher odds to one team. (For example, Bet $1 and get back $5 if team B wins)
These will both fluctuate before a game, as more people place their bets. The Bookie's goal isn't to correctly predict the game, but to end up in a situation where their risk is minimized, with roughly the same amount of money at to be paid out regardless of which team wins.