Two reasons I can think of:
1) Formatting usually loses a chunk of storage space.
2) Marketing from HD manufacturers. Be aware that:
1 KB = 1024 bytes
1 MB = 1024 KB
1 GB = 1024 MB
At their convenience, some companies advertise 1 GB = 1000000000 bytes, but in reality, it's 1073741824 bytes.
If you recall the first 1 GB HDs, they were often advertised 1.08 GB.