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 1000MB, 1MB is 1000KB and so on.
Software or computer language recognizes space in terms of power of 2 say 21, 22, 23 etc.
Hence a KB is 210Bytes which is 1024 Bytes.
Lets do a little math: From a manufacturer a 250 GB Hard Disk 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) =232.83gb
Now you'll see why your 250gb hard disk has only less than 232.83 GB space in total.
Well if you go with higher space like 500 GB and so on the space missing is increasing.....!