Why is Apple more than T-Mobile

I have an iPhone 11 Pro with Go5G Plus as my plan and I want to keep it. I was looking to get the 512 GB iPhone 18 Pro Max, but why I am being offered different rates from Apple and T-Mobile? I know about the $35 device connection charge, but that's not included in the monthly rate anyways. Is it because it is automatically changing me to the Experience Beyond plan? I want to stay on my plan, but I do not see an option to switch or keep plans when I am checking out on T-Mobile or even anything about plans. I was looking through the documents and terms of agreement and Apple is offering me $400 for my trade-in, but T-Mobile is offering $500. But I thought $500 for was only for the Experience Beyond or Go5G Next. I am very confused. I am even logged into my T-Mobile account and attached my iPhone 11 as trade-in. Does anyone know what's going on here?