This is true - but in an ideal world, if the bookie (a non-gambling bookie) had his choice, he would have the exact amount bet on both sides of a game and make a guaranteed profit on each and every game.
But, as you point out, the public will tend to bet on one side or the other of a line - so that creates the 'variance' for the bookie, which is when he has his 'good weekends' and his 'bad weekends'. I think in theory that all washes though - i.e. sometimes the public is right and sometimes its wrong.
But the bookie/sportsbooks want the bets to be equal - that is why the lines move. To try to get people to bet the other way when one side gets bet on too heavy.
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