as thymanbearpig said the APU using some of system memory as video memory..
edit: i don't know a way to set the memory igpu use, maybe there an option in bios maybe there aren't,
maybe disable cfx reduce the vmem.. (don't have any experience in it)
anyways i think, don't need to do anything, the benefit for gains more memory by making the vmem smaller
may not as big as you think, and there will be downside too.... (your apu/igpu crippled)