It's not getting your premium back because you didn't need it. You'd still get back this portion of the premium even if you get into a car accident on your way to the grocery store, because it's not about you, it's about everybody else. The average risk went down, so the average premium went down.
This certainly happens in the other direction too, they'll raise your premium for the next period if the average risk goes up. But their marketing departments aren't stupid. Sending everybody a check right now makes their "largess" more conspicuous to customers than sending everybody a smaller bill next quarter. It also keeps you from canceling your policy with them and switching to someone else currently offering a lower rate because of the currently lower risk.