80GB = 80,000,000,000 Bytes
80,000,000,000/1024 = 78,125,000 Kibibytes (KiB)
78,125,000/1024 = 76,293.945 Mebibytes (MiB)
76,293.945/1024 = 74.505 Gibibytes
Computers incorrectly report GiB (IEC Binary 2^30) as GB (SI 10^9). HDD manufacturers have it right, a Gigabyte DOES equal a billion bytes.
The confusion comes from assigning the term "Kilobyte" to a unit of 1024 bytes. This is WRONG. Kilo is an SI prefix that can ONLY mean one thousand of something.
If we'd settled on using binary prefixes back in the day or, like Snow Leopard does now, reported capacities correctly then all this hassle would be a thing of the past.