Good question charles_scott. I just know that when people advertise drives, when they say 80 GB, they really mean 80 billion bytes, which is a bit different. Since you go bytes -> KB -> MB -> GB, 80 billion/1024/1024/1024 is how much you should have in GB (it equals something close to 74.5 GB).
As for your 78 GB total... for some reason they gave you 83.75 billion bytes perhaps!?
As for your 78 GB total... for some reason they gave you 83.75 billion bytes perhaps!?