I've owned 5 cars at one point, and dropping or adding cars never changed the amount of the premium attributed to the other cars. (That is to say adding car 5 w

I've owned 5 cars at one point, and dropping or adding cars never changed the amount of the premium attributed to the other cars. (That is to say adding car 5 was an increase similar to adding car 4, for example.) I never understood why, seeing as how I can only drive one at a time, but I've always just assumed this is customary.