A poll takes the opinion of a random selection of the population. Pretty difficult to cheat. Bookies set their odds mostly on the basis of how many many bets are made each way, taking into account bet value. All it would take to shift the odds would be someone (or a group of someones) willing to spend a few million in bets to one side. Cheaper than advertising, a good way to sway the gullible, and if it works they get more than their money back.
