The single biggest factor that determines the point spread for a Las Vegas sports book is the bets that are coming in. If a lot of people are betting on LA, then the spread will be adjusted in LA's favor. The bookmakers don't care about the outcome of the game, nor are they trying to predict the outcome. They are trying to maximize their own profits.
Ideally, they would like to balance the bets so that the number of bets for LA is about equal to the number of bets for Boston (number of bets places, not the amount of dollars wagered). They will adjust the spread to try to achieve that equality, for their own business reasons.