I can't check the file size (I'm at work), but that is normal behavior for a torrent download. Unlike other methods of downloading (ftp, even kazaa etc) torrents do not download the data sequentially. They just grab whatever bits and pieces are available and put them in order later. Some torrent clients will allocate the disk space ahead of time though, so the file looks complete (1 GB) right from the start, but is filled with empty data. More often than not a file is totally unusable until the download is 100% complete.

So in this case the other vobs may have already downloaded the data at the beginning of the file that gives your dvd software information on how to play the file, but that one hasn't gotten the beginning data yet.