1GB = 1,000,000,000 bytes (how HDDs are manufactured)
1GiB = 1,073,741,824 bytes (how software actually sees and uses HDD space)
40GB = 37GiB
80GB = 74GiB
100GB = 93GiB
120GB = 111GiB
160GB = 148GiB
200GB = 186GiB
250GB = 232GiB
300GB = 279GiB
320GB = 297GiB
400GB = 372GiB
500GB = 465GiB
750GB = 697GiB
1000GB = 930GiB
so yeah.