From the Help Center when you search for moved backward in on Wish List:
Why did I move *backward* on my Wish List?
This happens when another member who was "ahead of you" in line previously (and who got an offer of the book already) gets back into line, for one of several reasons:
The member's Wish List book never arrived, and was declared lost in the mail
The member's Wish List transaction was canceled by the sender
The member had had her account on hold, and has now returned from vacation and unHeld her account. She will go back into line at her previous position when she does this: if she was "ahead of you" before, you will move back a space.
Please be assured that we would NEVER put another member ahead on the Wish List without good cause--in order to go ahead of others on the Wish List, a member MUST have Wish Listed that item earlier (on an earlier date) than they did.