Anything that's not an Application, Music, Movie, Photo or Backup is counted as Other. As well as anything NOT indexed in Spotlight. If you have Spotlight turned off for a disk or folder it will count as Other.
If you are still uncertain that you have hidden junk somewhere, download GrandPerspective. It'll give you a visual account of all your files.
Oddly enough, I also think images may come in as other if they are inside an aperture or iPhoto library. At least mine does.
Maybe it's raw, maybe it's aperture itself? Don't know.
If they use iPhoto check whether the iPhoto library is covered by the photos portion or not.