Win 7 Pro 64-bit sees reduced size of hard drives

I just bought a Lenovo m58e desktop, model 7268, refurbished. Has 320GB hard drive, Core 2 Duo 3GHz, with Win 7 Pro 64-bit. Works great compared to my old P4 machine. Hard Drive is a Seagate ST3320418AS Barracuda.

But the hard drive shows up as 305GB only, even after a low-level complete wipe (writing all zeroes using the OEM Seagate Seatools for DOS boot CD, then that tool's "set size to native maximum" -- both operations reported as successful). By this I mean the Win 7 Pro x64 install CD sees the drive as "Unallocated space" only 305GB not the full 320GB, before partitioning and before the install.
Also, SeaTools saw the full 320BG and tested the whole drive "without error" (their Long Test).

So I swapped in a Western Digital Green Caviar 1TB. I made a partition of 250GB, but Win 7 installer showed it about 15 or 20 GB less before successfully installing the OS.

Is this normal? Does Win 7 pro create a hidden partition for system files?
I did see a 100MB "System Protected" partition, but no sign of the rest of the 15 or so GB.
(I used several 3rd party partition and data recovery programs to look at the drive with the refurber's Win 7 Pro install, and something was there outside of the visible partitions, but did not make sense to me.)
:confused

 

AHA!! Thanks for the explanation. Makes sense now. No missing capacity, just different methods of calculation.