well dont worry they gave you more ram than the card can use. its a marketing thing ppl will think its better and pay more for the extra ram but it will not benefit you
your card is not powerfull enough to take advantage of that much memory if that makes more sense
I dont think there are many people out there that get the correct vram amount with dxdiag, it is usually way off the mark... but if GPU-Z shows that the card has 1gig of vram installed then it really has 1gig of vram.
lol if you are bored you could count the vram packages on the board and google the model number to find out how much vram each one of them holds. (ram packages are flat and black square components around the gpu chip itself, depending on the type of cooler used they might be easy to see (or not))
for example
This GDDR5 memory chip is made by Samsung and the model number is K4G20325F0-FC03.