I'm not sure if you've ever noticed but the .ftmb file generated by using "backup" is actually a zip file with relative addresses in it, i.e. no absolute path information. It contains the .ftm file and all the media files. The Autogenerated .ftmb contains only the .ftm file.
Probably to reduce errors all media files that do not live within the standard folder are copied as though they are in the standard folder (not the media folder) and then saved in the zip file - it can be opened using 7-zip, amongst others. I think this means that every .ftmb has its own copy of the media files.
Even if the media files are kept in a folder on another disc, they will be copied across to the "standard" folder and saved in the .ftmb file and the .ftm file "adjusted" to point to the new location before being saved as well...
[standard folder on my machine is My Documents\Family Tree Maker]
That's what my testing a few weeks ago showed. I expect it's to stop errors when the .ftmb is copied to a totally different computer.
It's possible to have several folders within the main media folder and put the various images in those. They'll be saved in the .ftmb retaining that relative path information. I haven't tried this so I'm not sure what happens on restoration.