Without knowing the Make & Model of your Motherboard and APU Processor generally most Motherboard BIOS Settings has a maximum vRAM for its Integrated Graphics at 2 GBs of vRAM which is assigned by BIOS and taken away from your System RAM Memory total.
Some Motherboards has higher vRAM Limits than 2GBs in BIOS Settings. I know some Asus Motherboards has a Maximum vRAM of 3GBs for instance.
So it seems like your Motherboard BIOS Setting is set either to its Maximum amount of vRAM or you need to enter BIOS and see what is the maximum amount of System Ram you can assign to your APU Integrated Graphics.
NOTE: Windows will automatically assign what ever System RAM your game needs to run.