Because not every bettor is totally analytical about how they place their wagers, with it being fairly common for fans to want to put down a couple of bucks on their favorite team regardless of the odds or their chances of success, popular teams can wind up with shorter odds.
The Red Sox and Yankees might both have a good chance to win the World Series in any given year, but there also will always be more money bet on them because of their popularity, thereby shortening their odds as bookmakers look to reduce their exposure.
What this means is that there might be better opportunities, both on those long-range bets and on MLB daily lines, when you peruse the odds for less widely-followed teams. If the Chicago Cubs and Milwaukee Brewers have about an equal chance to win a big game, the odds will tilt shorten on the Cubs because more money will be bet on them.
Savvy bettors -- or "sharps" as they care often called -- will be aware of this, ignore the shifting odds, and look behind the numbers to see two teams with equal chances, but a better payout for Milwaukee. That’s an important thing to keep in mind in a public betting market: That it involves the public.