windows xp - How is my system capable of using more RAM than stated in its specification?

I have a Dell GX520 Optiplex model (mini tower) and upgraded the RAM from 1GB to 4GB. Although the Dell specification states

2 DIMM slots Non-ECC dual channel shared DDR2 SDRAM system memory (400 and 533Mhz) up to 2GB.

the Dell and the operating system have no problem using the whole 4GB.

How can this be happening? Are the specifications not accurate? If I was to upgrade to 8GB could it still use all of the RAM?

Answer

That model will take 2GB in each slot, for a total of 4GB. Check the manual.