I guess the theory is that they exumed her at that point from the first location, and then moved her to a place that had already been searched.
Yes.
or the area they found was a grave in waiting, but that the area where he first buried her (if she is under this mass of water) was too to hot or it filled up with water before they had a chance to move her to the grave in waiting.
Or the "grave" they found wasn't really a grave or it was not a grave in waiting meant for Natalee.