you've already had this explained to you.
the 16GB on the box of a Hard drive or flash drive refers to a GB as 1000MB. The computer works in binary so a GB is really 1024MB, which is 1024KB, which is 1024 bytes.
so your 16GB drive is 16,000,000,000 bytes, which is 16,000,000,000/1024=15,625,000 Kilobytes, which is 15,258.7 MB, which is 14.8 GB.