Charging me full rate for replaced vehicle

I recently paid the remaining 3 months of my 6 month insurance premium off. Then I traded in my car (didn’t expect to do so) and replaced it. The 6 month premium went up $900. That part I understand as it’s a newer car. However, they are charging me a full month’s bill rather than the difference I owed. So for example they are charging me $420 when the previous payments were $290.

Before I call, I just want to make sure I’m not missing something here