From Wikipedia:
Quote:
To further complicate matters, flash memory chips are organized in multiples of 2, but retail flash memory products have available capacities specified by multiples of 10.

There has been lawsuit cases about it, like this one against WDC. Pretty much the result has been every storage product now has tiny print saying 1GB = 1,000,000,000 bytes. It's even in small print on the back of my iPhone box.

8,000,000,000 / 1024 /1024 / 1024 ~= 7.4GB. OS size, plus probably a bit extra for future growth accounts for the 6.9GB usable for storage.