Cancellation dates and insurance coverage dates may not coincide, but if you are on a monthly payment plan your insurance should have been paid in advance.
Sometimes the billing on your policy is just not calculated correctly by the company.
How insurance companies calculate your monthly insurance billing can be very confusing. First, they usually want you to pay for two months down when you start. This is because with monthly billing plans the insurance company is going to have you pay a month in advance.
Second, your billing due date will most likely not coincide exactly with how much insurance you have purchased. For example you may make your payments on the 1st of the month, but your monthly insurance coverage may run from the 20th to the 20th if each month.
Still, being on a monthly payment plan you should be paying in advance. If you have cancelled your insurance and the insurance company is saying that you still owe them money then one of two things may have happened. One, the billing system does not have the correct or current cancellation information. Did you get a cancellation notice from the company to verify cancellation and what date did it have on it?
Two, the billing is not accurate. You need to contact the insurance company and ask why you are still being billed for a cancelled policy. If they tell you that you owe them for insurance coverage prior to your cancellation date, challenge them on it. Make them provide you with a breakdown of all the payments that you have made and the time period of coverage that the payments covered.