I believe that is due to the fact that hard drive manufactures round done the size's of GB to 1,000,000,000Bytes when the true sixe is 1,073,741,824(1024B=1KB, 1024KB=1MB,1024MB=1G
. So your drives true size is 37.9GB. I guess they due this so the drive capacity comes to a round number.
*note are you sure your drive isn't a 40G as this would translate to 37.2G