It's not a site "bug" per se. It's a side-effect of how the database is constructed.
The problem is that each movie file and each avi file have their own entries in the database. If a movie has two movie files (such as is the case with
http://tasvideos.org/83M.html) then it has two entries in the database. Likewise, if a movie has two avi files (such as is the case with
http://tasvideos.org/817M.html) it also has two avi entries in the database.
Some statistics require these to be separate (eg. the file size of the avis is different and thus need to be handled separately) while in other statistics it would be better for them to be merged into one (eg. the number of rerecords doesn't change from one avi to the other).
The routine which handles the statistics is generic and by itself cannot make the distinction whether two entries should be merged or not. I suppose it could be possible to parametrize this and make the routine to merge movie/avi entries when requested, but I haven't looked into that yet.