Dxdiag is telling me my Radeon HD 5770 only has 626 mb of VRAM?

I'm getting a confusing reading from dxdiag, and I think it's
preventing me from playing a few high performance games (Skyrim D
Well, ny AMD Vision Engine Control Center is telling me I've got 4735
mb of VRAM, while the dxdiag is telling me I've got 626. Any ideas?

If your running dxdiag while playing a game, it will in most cases
display less memory than whats installed in your video card.
If not, the memory on your vid card may be faulty.

A bit of additional info...
First of all, the 5770 has 1GB of GDDR5 memory on-board, but is able to
share unused system memory. ATI CCC will show how much of the shared
memory is available to the vid card, thus the 4735mb value. Dxdiag
however, will only show the amount of memory on the vid card itself
e.g., 1024mb, but may show less if running dxdiag while playing a game
e.g., 626mb.