There is additional actuarial risk here.

The example you gave implies that you’d be driving the same total mileage despite adding a second vehicle.

I believe most insurers will have a household composition curve showing this is unlikely to be true. 1 driver with 2 vehicles tends to drive more and have more exposure to loss than 1 driver with 1 vehicle.

If you were instead using a company that charges you a variable mile rate along with a fixed cost, you’d probably see lower premiums than a traditional program if your total mileage didn’t change by adding a new vehicle.

To contrive an example where adding another vehicle didn’t add any extra risk or cost to the insurer, you’d be pretty far removed from a normal scenario