Could be a bug. There used to be (or maybe there still is, I don't have a Radeon HD-7000 card anymore) a bug that reported the wrong amount of memory to Windows, even though everything was working fully. Big question is - does it work properly? Are all GPU's being used?
i just run furmark and hitting 105fps .. there is probably something wrong with the pci lanes because it use to be 120fps with this cards using x58 Rog motherboard.