Do you believe on formula 100 GB = 93.1 GB?

I bought Toshiba laptop with configuration of 100GB Hard Drive. When I checked the size of the hard drive, it was showing around 93 GB? Where the rest of the space went? or Toshiba is cheating the customers??? I applied some maths on the calculation and figured out where the rest of the space went.

100 GB = 100,000,000,000 Bytes
but computer actually calculates as
100 GB = 107,374,182,400 Bytes because 1KB is not actually 1000 Bytes but 1024 Bytes
so 100,000,000,000/1,073,741,824 = (approx) 93.1 GB.

This is true not for only Toshiba laptop but with almost all computers advertisements. So be aware.