Right off hand I can think of a reason that would happen. I'm not saying that this is the case with your DVD, but a DVD can be full, and files not show. I have burned DATA DVD's and put all files on there as HIDDEN. Therefore, the DVD is full but the files won't show in windows unless you have windows set to show hidden files and folders.
Another reason can be that the DVD is a DVD-RW and has been formatted for packet-writing. If it's a RW disk, try erasing it. Even then, some RW's can get corrupted and show as full when there's really nothing on them. In that case, I just throw them away. Not much else you can do. I have ran across many DVD RW disks that do what yours is doing. Basically, they were just F^cked up, and are food for the garbage can.