Operating system may display incorrect amount of RAM due to many reasons, one of them is if you add more memory then BIOS will recognize the full amount of RAM but sometimes Windows recognizes only a part of the RAM. This is due to enabled redundant memory feature a memory mirroring feature in the BIOS. Both features should be disabled but they can only be accessed via BIOS not via Windows.
It can be corrected as Go to Start > Run > MsConfig then boot tab then click on advanced option. If max memory option is checked, then uncheck it and then reboot.