Charged after Cancellation

I cancelled my internet services in late July, on July 21st, and turned in all of my equipment. My internet billing periods have been from the 10th to the 11th of each month, and I paid my July bill in June, meaning I would have gotten a prorated refund since I cancelled with over two weeks left in my billing cycle.

Now I get a bill at my new home address asking me to pay a bill for July 20-August 21. 1. I already paid the bill up until August 10 and didn't get a refund, so why do I have to pay for the additional days until the 21st, when I had not been using Xfinity for over a month by that point? I received this bill on August 21st