DirectX 11 and Sapphire Radeon HD 5870

My graphics card is a DX11 card and as so I have enabled the D11 setting in-game. It keeps the setting for some days, but suddenly out of the blue, it just resets to DX9. My game runs smother in D11 so I have no interest in running it at DX9. Why is it that it randomly resets back to DX9?

 
FAIL (the browser should render some flash content, not this).

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