Betting odds vary from website to website more than most people think. There are many reasons why odds may vary between sportsbooks, but the main causes are:
1) Different profit margins
2) Too much liability on one side of an event
3) Some sportsbooks are slow to react to news (eg. injuries, lineup changes)
The difference in profit margins is most noticeable at the beginning of the day; opening odds vary less than closing odds so a lot of the difference is caused by the books' margins. If you're looking to take advantage of differences in odds, some of the best opportunities arise closer to the start of the event.
Throughout the day, public money might bet on one side of an event on Website A more than Website B. With too much potential payout on one side of an outcome, Website A may be inclined to change their odds to make that popular bet less attractive. Small shifts like this happen all the time on lots of different books. Crowd favourite teams may be overbet for a big game, pushing too much liability on one side. Ironically, sportsbooks aren't in the business of taking risks; they like to try to balance liability to guarantee profit, no matter the outcome.
Common odds for a 50% outcome are 1.91 to 1. If you make 1,000 wagers at 1.91 with a 50% chance of winning, you'll need to win 52.4% of the time just to break even. If you can find a line paying 2.02 for the same 50% events, you'll be profitable with anything over a 49.5% win rate. Betting on underdogs, you can frequently find lines that are paying significantly more than their "true odds" would suggest. It's not uncommon to see an event paying over 4.00 while most other sites are paying closer to 3.60. SmartLines makes finding these money making opportunities easy and efficient by aggregating odds from multiple books to make you a smarter bettor.
The graph below shows 1,000 simulated bets with exactly 500 wins and 500 losses and a starting bankroll of $100 making $5 bets. The red line shows what happens to a bankroll taking odds of 1.91, while the blue line bet on the exact same outcomes but got 2.02 per bet.