Ha!  Funny you should say that.  There actually IS a standard.  It's a matter of who's standard you believe in.
HDD manufacturers like to use base 10 math, so a gigabyte is calculated as 1,000,000,000 bytes.  The confusion arises because computers have always used binary, or base 2 math.  That same gigabyte is then calculated as 1,073,741,824 bytes.  Add to that the fact that formatting a drive consumes additional space.  In the end, that "500GB drive" only gives you 400-and-change GB.  Apple even got sued over this nonsense.
More details on this...
http://blog.tweefari.com/apple-mac-os-x-106-snow-leopard-uses-base-10
http://support.apple.com/kb/ts2419
Though it seems this changed in SNOW Leopard (10.6.x) whereas you're running the previous, regular Leopard (10.5.x).  Perhaps it changed in a subsequent update to make the two behave similarly...