It's marketing.
60G = 60 000 000 000 in the normal world. But since the factor "1000" in for example 1kB is actually 1024 in the computer world, you get these missleading figures. I.e. 1kB is 1024 B, not 1000B.
So, 60 000 000 000 bytes divided by1024 to get kB is 58593750kB, which divided again by 1024 gives you 57220,5MB and divided by 1024 again, 55,8GB.
It was decided at some world computer meeting back in -95 that the computer meaning of kilobyte (1024 bytes) should be written kiB, 1024 kiB should be written MeB and so on to avoid confusion. But it never caught on.
/Ola