Mathematics and statistics. Liverpool have been overperforming in terms of expected points and goals, whereas City have been severely underperforming in the same departments. Bookies are mathematicians at the end of the day and they are expecting a return to mean this season, as they have been every season.
Me, I like to think both, and I'm happy with the underdog tag, always have been. Gives me good odds. There's math, and there's mentality, and there's teamwork, which don't seem to factor into odds;)