GM files in Halion SE

I use Cubase 6 with GM files which are useful for my choir as they can be edited quite easily.

Some general midi files ‘tell’ Halion SE which sound to use, load them up, and I can use the file immediately.

However, some files, even though the instruments are listed, don’t do this so I have to physically put them into the Halion SE box which although easy enough does take time.

Is there any reason why some files do this?