Trade in

I traded in my son's iPhone 13 pro. I verified the condition of the trade in which specifically asks: 

Does the device meet the following conditions? 

-The device stays powered on.

-The screen on my device is free of cracks. 

-The device has never been damaged by liquid or moisture. 

My device did have a cracked back glass however this wasn't one of the requirements as per T-Mobiles condition requirements. If it would have been taken into consideration and the value of the device would have been lower then I probably wouldn't have upgraded. 

Why am I being charged the adjustment fee?