On our system, a gigabyte is 1,073,741,824 bytes. Hard drive manufacturers would have you believe that it's 1,000,000,000 bytes, but we don't sell hard drives.
Technically 1,073,741,824 bytes is a gibibyte, but we have a hard time calling it that without a childish urge to snicker, and nobody likes a snickerer. If you want to be strictly accurate, the hard drive manufacturers are right, and we bill storage and bandwidth based on gibibytes* and mebibytes* (1,048,576 bytes), not gigabytes and megabytes.
Someday we'll grow up about this, but that day has not yet arrived. We are moderately more mature when it comes to the abbreviations, and we try to use MiB and GiB to represent base-2 prefixes correctly wherever possible.
*Must... keep from... snickering.