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...