Payment not allocated on time

Good day I made a payment on the 27 of July 2023, to a game account, and they didn't allocate the payment on the date of payment and still charged me interest. Whe I enquired this is what they say"Good morning Ms.Rofhiwa Khampha,

Thank you for your email,

Kindly be advised the payment made 27/06/2023 of R 2910.00 was received but was allocated to your account after statement month end ,therefore will reflect in your next statement month end 10/08/2023.

Should you require any further information, please feel free to contact us on ***or via e-mail ***

Regards,

Shirley."

I don't understand how I'm liable for the interest, when I have paid on time.