"Each device in the system is assigned an address, which takes some of this space. Because the address space is limited, the more hardware you have, the more space is required to address them. Memory (RAM) will get what is left over, and it is this is what takes usable memory in a 32 bit system from 4Gb of RAM to around 3.25Gb."
So, the addressable space being used by your devices is taking part of your RAM. Why your machine is using 2 GB RAM for address space, I am not sure. Maybe I am not even pointing you in the right direction. Maybe someone else that has more experience can confirm this for you.