All Hard Disk manufacturer way of counting space is different from the way a software count a space.
This means to a Hard Disk Manufacturer, 1GB is 1000 MB, 1 MB is 1000 KB and so on.
Software or computer language recognize a space in terms of power of 2 say 21, 22,23 etc.hence a KB is 210 Bytes which is 1024 Bytes.

Lets do a little math:From a Manufacturer a 250 GB harddisk is 250 x 1000 x 1000 x 1000 =250000000000 Bytes From the Computer Language point of view a 250000000000 Bytes is:250000000000/(1024*1024*1024)=232.83

GB Now you’ll see why your 250GB harddisk has only less than 232.83 GBspace in total. Well if you go with higher space like 500 GB and so on the space missing is increasing....