What your Arena rating actually measures
It is not how often you are right. It is how often you are right about things the model was not already sure of.
Everyone starts at 1000. A settled pick moves that number by a single formula: the K factor, times the difference between what happened and the probability the model gave when you made the pick.
The second half of that sentence is the whole design. You are not scored against the field, and you are not scored on a hit rate. You are scored against the model's own opinion at the moment you disagreed with it - or agreed with it.
What that feels like in practice
Backing an eighty percent favourite that duly wins earns you very little, because the model already said so and you added nothing. Backing a thirty percent call that lands earns you a great deal more. And missing that eighty percent favourite is expensive, precisely because it was supposed to be easy.
This cuts in an uncomfortable direction and it is meant to. A run of safe picks on heavy favourites will produce an impressive-looking hit rate and an unremarkable rating. That is the correct outcome. If a number can be farmed by picking the obvious, it is not measuring judgement.
“A hit rate tells you how often you agreed with the favourite. A rating tells you what your disagreements were worth.
The details that stop it being gamed
The probability is frozen when you create the pick, not looked up when it settles. Prices move as team news lands; if the rating read the later number, an early pick would be scored against information it never had.
The K factor starts high and falls as your season fills up - forty for your first handful of settled picks, then twenty-four, then sixteen, then twelve. New accounts find their level quickly; established ones stop swinging on single results. Ratings cannot fall below 800, so a bad afternoon has a floor.
One pick per market and period per match, and the market you picked is what settles it - the same reconciled probability the board showed you, not a different number pulled at settlement time.
If you want the rating to move, the way to do it is to disagree with the model and be right. There is no other route through it, which is the point.