Why Tmobile didn't follow the promise which made while ordering the phone and later telling this offer is not applicable to you?

On July 20th , T-mobile website showed me $800 promotional trade-in offer for device upgrade(without adding a new number). I called the customer care and they did upgraded my device and they charged $35 for upgrade assistance, they also confirmed I am eligible for $800 promotional trade in offer for device upgrade( without adding a new number). I confirmed two times in the call that I am eligible for $800 promotional offer and they confirmed it and said making a note in the call log also.  But now T-mobile website is showing $400 promotional offer for my upgrade and customer care team is telling only $400 only mentioned in the call log instead of $800. I requested them to check the recording that taken while I gave confirmation for phone upgrade to the customer care person, but they said they couldn’t go back and check the recording. Then I am not sure what is the purpose of recording the conversation and why the customer care person was lied that $400 was noted in the call log instead of $800.