It's the same as bookies, right? They are often seen as predicting the odds of an event happening, but that's actually the odds they estimate so that the bets are roughly divided 50/50, to limit their exposure, which is pretty different and depends on their market. English bookies will always overrate the chance of England winning at football, because English people will disproportionately place a bet for their team to win.
I don't think it's that simple - if other people find out English bookies overrate England, then they make a profit by betting against. I'm not saying every market is perfectly efficient but it would be surprising if a major inefficiency lasted for a while. And if "enough" bets are placed the set odds will always be an unbiased estimate of the true odds.
Sports bookies have a significant informational advantage over the everyday gambler; they index massive amounts of historical data and have access to live data which is lower-latency than radio or television broadcasts. That helps make the market inefficient.
