Why 1TB external disk only showing 931GB in Windows?

I bought a 1TB external USB drive, but it only shows 931 GB in Windows 7.
This is due to that  drive manufacturer count TB in different way from Windows.
For Drive manufacturer:

1TB=1,000,000,000,000 bytes

For Windows:

1GB=1024MB

1MB=1024KB

1KB=1024 bytes

i.e 1GB=1073741824 Bytes

931GB= 1,000,000,000,000 bytes

So drive manufactures do not cheat us, but misleading. The definition in Windows is closer to the definition in computer science.