Then I dunno what is this. With the last one I meant that maybe you can't see some applications running in the task manager because you don't have rights, but I am not sure about this either.
The integrated graphics card on your motherboard (not the rx460) can use a great amount of memory if it is turned on, but usually if you have a dedicated vga, then it is off. Anyway if it is turned on then usually windows shows max - vga memory as the total memory.. so if task manager shows 8gb then probably it is turned off, but you can check that in bios.