There are two reasons: Limitation or Compatibility
Limitation
32bit Windows can only recognise 4GB of RAM. Depending on the bios it can be as low as 3GB. If you ahve installed 4GB or 8GB and the system is only recognizing 3GB it is likely your system is 32bit.
Compatibility
Not all computers are built to recognise every size module. Some manufactures have created a hard memory limit. If this limit is 4GB and there are two slot, then the maximum RAM size for each slot of 2GB. If you have installed higher capacity RAM and you are getting unusual results, it could be those modules are not compatible. Please check our configurator for compatible modules.