Hm. We're querying the AMD driver directly for your total video memory and DirectX for the amount of memory used. I'm guessing AMD's tools must have access to better data that isn't exposed to us for some reason.
I suspect you are in fact using up your physical video memory on the card, but there is also some shared memory in the picture that we don't know about but the AMD tool does. It's probably tight, but if you're not seeing any performance issues, it's safe to ignore I'd say.