Because the producer regards 1Gb RAM as 1000,000,000 Bytes, whereas your machine will regard 1GB as 2^30 Bytes. As a result, the RAM may appear to be smaller than it should be.
Also, you might have been careless when u installed the RAM, so that some small part of your RAM is damaged. This also leads to lower RAM amount as well. But this effect is normally trivial.
DFI Lan party UT NF4 Ultra
AMD Opteron 146 2.0 Mhz @ 2.8 GHz
OCZ Gold Edition 2x512 MB DDR 500 timing 2.5-3-3-8 @ DDR 560
XFX GeForce 7600 GT 256 Mb GDDR3
Antec Lanboy Mini Tower Case
Antec True Control 550W ATX2.0 PSU
Maxtor 300 GB 7200 RMP SATA 16Mb Cache
17\'\' TFT Screen 12 ms