It's the same as HDDs... that'd be 8GB of "unformatted capacity". When you format it's pretty much like a HDD, ie 8GB = 8,000,000,000 bytes, or 7.45GiB (divide by 2^30). Subtract from that the OS (ISTR it's about 400MB?) and some databases and 6.91GB sounds believable. The chips are sold as 8GB chips by every NAND vendor.

It's always been this case for flash, have a look at the actual capacity of the average 2GB SD card, for example.

It's not so easy to "top up" the capacity - that'd be adding another chip to the board unless every NAND vendor - Samsung, Toshiba, Hynix, Micron, Intel, etc - agreed to add capacity to their die. Nobody is going to go out on a limb and do this in isolation, as it makes their NAND more expensive to make compared to the competition, and they're probably not exactly making piles of money on the current pricing...

Hugo
(speaking for himself, not anyone else)