[user104658]

Betti g odds are (in large part) based on what people are betting on which doesn't necessarily reflect the likelihood of actually winning, especially when the voter pool and betting pool are different.

I.e. These odds are coming from the bets of the British public, who have a large component betting patriotically, but they have no actual sway over who will win, so the odds can skew in ways that are incongruent with the likely outcome.