I think this prediction is limited to their next match, not to who will win the Premier League. That being said, it is still unclear how Google calculates the winning probability for each match. There is a suggestion that they use historical stats, others predict they use aggregated betting odds from various gambling websites, and so on. I even suspect they use Opta's LWP model to some extent. So yeah, it is still an unreliable metric and you should not use that as a basis to make any serious prediction at all imo.
I predicted based on my personal analysis and I don't follow the trends made by the public. We predict based on the status of a club, you don't go about betting in favour of Luton Town to win an elite club like Manchester City, it's impossible except you want to risk your money, usually the odds that follows becomes significant. The prediction of these clubs are usually based on their previous statistical data and I must say, these clubs brings shocking and amazing scores. You're right, before a game is started, we have see the odds of the losing and winning club.