T Mobile Apple store promo

There is a T Mobile promo listed on the Apple Web site.  Both Apple and T Mobile pitch in.  I need clarity on the process.  

I selected my phone, listed my trade in, and it said I qualified for $ 800.  Then before completing the process it says $ 400 down, or $ 600 down, and shows a high financed amount.  Why would there be anything down, or anything financed it you get $ 800 trade in, and select an $ 800 phone?

I have ™ internet and plan to switch from ATT to ™.  Don't want to make a costly mistake.