Billing and Discontinued Service

Good afternoon. 

I had been a happy xfinity client for nearly 11 years - after trying to change my plan repeatedly, I ultimately discontinued service. I paid my final bill on 7/2 - and cancelled service on 7/6, scheduling for disconnection on 7/16.  I understand that my payment on 7/2 is for what my statement reflects is service between 7/16 - 8/15.  I am expecting a full refund of this pay period (confirmed by a call center agent as well as an in person agent) - when questioning this through customer service, I was told that instead of advance payment, my 7/2 payment reflects service between June - July.  Everything in my research suggests that Xfinity participates in advance billing and thus I should be issued a refund for the full amount.  I'm confused - and the texting agent wasn't helpful - could anyone confirm?