interesting, but I would not use the 7k display as comparison.
various points:

I assume the cards format as FAT16 whereas floppies are FAT12, so differences between numbers of small and large files may build up?

Cards are specified as MB = 1,000,000,bytes like every similar medium like hard drives, whereas windows and ide channels count a MB as 1024*1024 = 1,048,576 bytes so an apparent reduction in size always takes place due to this with every medium.

The filesystem must have an overhead, if this is relatively constant, it will be advantageous to the bigger cards and disadvantageous to the smaller.

We shall see how filling the filesystem with playlists etc affect the capacity, or whether this is already spoken for in the overhead.

5448 kbytes free on the 7k shows as 5579 kbytes in windows. If you look at the exactly the same files on pc and floppy you will see a different number of bytes used due to above reasons.

It looks like the file system overhead is the majority influence.