I have therefore received a refund check from a previous car insurance company that said they had made me pay more for it. Is this normal?

Yes, it's completely normal.

Auto insurance, including its cost, is heavily regulated by state and provincial laws. And errors, including overload, occur regularly. If an insurance company charges more than it is allowed to pay, it must return the money to the customer.

Failure to do so would result in very severe penalties and judges do not like the insurance companies more than you and I do.