When the prefix matches a request sent to the same Providor. The thing is the TTL is different for each provider, some cache for 5 minutes some cache for 1hr. Its ideal to only use one provider per agent session / and per model with the best cache hit % if you care about costs.

But before 5m the hit rate is 100%, and after it's 0%? Why is there a probability?

Is there some stochastic process that takes place during those 5 minutes that determines whether or not you get the discount?

Presumably the number that OpenRouter shows is averaged across all requests.