I never can get the Numbers straight, but i think it can be summarized as having to do with one using 'GiB' vs the other using 'GB'
I guess you could think of it in a way akin to the Metric system vs the Non-metric sytem.
A quick snippit from wikipedia. http://en.wikipedia.org/wiki/Gibibyte
The gibibyte is a standards-based binary multiple (prefix gibi, symbol Gi) of the byte, a unit of digital information storage. The gibibyte unit symbol is GiB.1
**1 gibibyte =** 230 bytes = 1073741824bytes = 1024 mebibytes
The gibibyte is closely related to the gigabyte, which is defined as 109 bytes = 1000000000bytes, but has been used as a synonym for gibibyte in some contexts (see binary prefix.) In terms of standard gigabytes, 1GiB ≈ 1.074GB.
You often see people confused about the size of the Ubuntu ISO files, the size listed on a web site or ftp server seems to be 'bigger' then a 700mb due to how different systems use GiB or GB.