256MB of memory addresses assigned to 1GB GPU?

Hello,

Looking at my GPU in Device Manager and the hardware resources that have been assigned to the card, it only shows that 256MB worth of addresses have been assigned to the card (screenshot taken from PCIScope) -

My card is a fairly old Radeon HD 5850 with 1GB of dedicated memory so I'm just wondering why this is?

When I view the card's adapter settings it shows the correct amount -

Would anyone be able to explain why this is? Any help would be appreciated,

Thanks.