I did not say that it is not correct, just that it looks like it is measured differently. If you measure with one system in the display and another in windows or floppies the comparison will be wrong, that is the only point I'm making.

If the overhead is the major factor, then calculating bytes and different file allocation system blocks will be the minority factor and may be able to be ignored in the bigger picture, though it still contributes. It looks as if this may be the case here. We don't know what they have done in the overhead. There is presumably space for all the navigation features, maybe filenames, remembering space is needed for the structure of possible midi files and audio files as well as technics format and individual custom style saves as opposed to full customs, and also any pictures for slideshows and any playlists you may program. Maybe this size overhead covers all these loading structures and their possible filenames? It seems likely because you have alphabet sort available in the menus.

A very similar scenario presented with the SX hard drives, if you added up the free space shown for each partition and compared it with the manufacturers quoted size of hard drive.