why total capacity of hard disk sometimes is not equal with adding all hards capacities?

Answer 1

The hard drive space will never equal the amount is says you have. There are files that are installed on the hardrive to partition the drive. This amount of hard drive space you will never get back.

Answer 2

two reasons ...

there are two ways to calculate capacity - one is denary and one is binary - denary has a thousand as 1000 but binary has it as 1024

a completely blank disc has one capacity but it is reduced once it is partitioned and formatted (as this uses up space) - one of my 40gig drives turns out to have 32 gigs of useable space