Strange thing of my case

My I-140 was approved last November and my lawyer has received the Approval Notice from USCIS and then forwarded the notice to me. In USCIS website, I just happen to check the status of my case by inputing the case number. the strange thing is the returned feedback is: The notice about my case was sent back by the post office b/c undeliverable. I can not understand it. I did not move at all. And actually I have filed 485 and got the receipt and fingerprinting done.