I had two cars on my insurance. I deleted neither and added a brand new car

Theory 1: Those with spare cars tend to wreck less often than single-car owners.

Theory 2: Volvo owners tend to wreck less often, no matter whether they have a second car or not.

Theory 3: Rates are determined first from your primary car (presumably, the Volvo) and then tweaked for your secondary car. After all, you can only drive one car at a time, and you'll mostly be driving the primary car.

At the point that you sell the BMW, you'll get to test these theories. If your rates go up, then theory 1 might be true. If your rates don't change, then theory 2 might hold. If your rates go down, then theory 3 might rule the day.