Predictable unpredictable?

If the result of the Bristol mayoral election is really 'completely unpredictable' (as this Post story says) how come the bookies can come up with betting odds? Or is it that we know from past voting patterns and current polls and political developments that the most likely outcome is a Labour or liberal-minded independent candidate winning? In other words Marvin Rees vs George Ferguson because both are broadly progressive and not being blamed for current problems. Surely we can count out Tories and Lib Dems because of the ongoing recession and the cuts/austerity program. The Tories dont have the votes needed across the whole city in any case and the Lib Dems are taking the brunt of the criticism of the coalition govt (thus they are at 10% or so in the polls). Hard to predict the final result between two particular candidates: yes. Completely unpredictable: no.