19.5GB + 17.7GB = 37.2GB

Many hard drive makers define a GB as 1,000,000,000 bytes.
So 40,000,000,000 bytes divided by 1,000,000,000 bytes/GB equals 40GB.

Many operating systems define a GB as 1,073,741,824 bytes (2^30 bytes).
So 40,000,000,000 bytes divided by 1,073,741,824 bytes/GB equals 37.25GB.

Measured one way or the other, it's still 40,000,000,000 bytes. An analogy would be your height; whether measured in feet or meters, you don't get any taller or shorter.