Apple watch returned after trade in?

Hello,

I just purchased an apple watch series 6 and did the trade in on my series 3 for 100-150$ trade value. I received the box to send the watch in. I cleaned the watch and wiped the data from it before sending it out. I just received the watch in the same box back in my mailbox 2 weeks later with no note or any documentation as to why it was returned. I am assuming now I am not getting the credit towards my new watch and am paying full price. Which if I would have known this I would have waited or held off until the next generation. Why was my watch sent back? It works great and has a small scuff on the corner of the screen but otherwise in near perfect condition.