Metric defines it as:
1KB = 1000 bytes
1MB = 1000 KB
1GB = 1000MB
1TB = 1000GB
etc.
JEDEC uses a binary representation (which is how computers count it,) where:
1KB = 1024 bytes
1MB = 1024 KB
1GB = 1024 MB
1TB = 1024 GB
etc.
By the time you get to a terabyte, the metric representation looks something like 1,000,000,000,000 where the binary representation would look something like 1,099,511,627,776. So from 1TB that's ~92.67GB (or GiB if you will) that they essentially swindled you out of.
Now consider someone like me who has 4x1TB drives. In RAID-5 I only effectively see 2654.72GB of storage because of it instead of 3000GB (granted in addition to how it is counted, there is more space that is used for partitioning and file systems and such.)