Why do these pricing inconsistencies or under round prices occur?

As bookmakers can now operate online, there are now hundreds of online fixed odds bookmakers. They are all competing with one another and are located in various countries and time zones. To be the person who actually prices a particular sportsbook you have to have a massive amount of knowledge of that sport. To compete, fixed odds bookmakers offer loads of sports in their sportsbook, but there isn't the expertise to cover every sport. They may have the relevant stats but they can't possibly know every participant and every event. In short mistakes can and do happen. Taking this into account and the difference in time zones you get hundreds of opportunities for sports arbitrage trading every week.

Take two examples.

When England play soccer most bets with UK bookmakers will be supporting England. Bookmakers may offer the opposition at an inflated price to create a balanced book.

A typical week with four golf tournaments and a couple of tennis events bookmakers may have to price up 800 or so outright odds plus over 400 matches just in these two sports alone. We only need two bookmakers from the many available to have a difference of opinion in one of these markets to create an opportunity.

Back / lay sports arbitrage betting

You can also find a sport arb between the back and lay price (win / lose) for the event. Back / lay sports arbitrage betting has been made easier due to betting exchanges like Betfair. A betting exchange allows members to lay an event (e.g. for a horse to lose). A back / lay sports arbitrage bet occurs when the price offered by a bookmaker is higher than the lay price available on a betting exchange for the same selection. The difference between the two represents your profit. Often a bookmaker can be slow to change the price on a favourite when something has happened to reduce the odds. As the price drops on the betting exchange you profit by laying low with the betting exchange and backing high with the bookmaker.