iPhone upgrade program

I purchased my iPhone 14 Pro Max in September of last year on launch day under the upgrade program. The program says you have to make 12 payments in order to be eligible to upgrade. I made 13 payments because when I purchased the iPhone I had to pay the taxes plus one mont in advance. My twelfth payment was last month and my 13th payment is due and paid before launch of the 15 making 13 payments. Now when I pay for the 15 which will also be taxes and one month in advance this will mean that in September I made two payments on the program. Can someone make theirs make sense?