Any hard drive you buy is going to describe its capacity in decimal, while the computer uses binary, which is the measure you're used to.
1 GiB == 1,073,741,824 bytes (one binary gigabyte)
1 GB == 1,000,000,000 bytes (one decimal gigabyte)
1 Gib == 134,217,728 bytes (one binary gigabit)
1 Gb == 125,000,000 bytes (one decimal gigabit)
Nice and confusing, especially since most people just use GB with an arbitrary capitalization regardless of what they mean.