It's all dependant on the manufacturer of the flash memory. SanDisk creates and formats their flash memory/media in a different manner than what, say, Samsung creates and formats. Any 512MB Duo from SanDisk will give you 467/8MB maximum capacity no matter what.

The main reason, though, that you get less capacity is that these manufacturers base their storage media capacity on the fact that 1MB = 1000KB (or 1GB = 1000MB). This obviously conflicts with the true nature of the MB, it being 1MB = 1024KB (or 1GB = 1024MB). So right off the bat, you're guaranteed a loss in capacity.

Combine that with the obvious formatting of the memory/disk, and you're left with what seems to be a 5-10% loss in actual storage size (the percentage grows as you get into larger capacities).

And that's all I really know, unfortunately. :)