AMD HD7970 GPU

I wanted to ask about the benchmark specifically at first but after thinking on it my question might be more broad than all that.

As far as I know; DX11 is suppose to run smoother (a.k.a. easier) on systems that support it as compared to DX9.

My benchmark scores on dx9 were 2k over my dx11 ones. Still dx11 was a par 7k but I can't help but wonder why. My system runs dx11 fully as far as I know.

Settings for both benches were max as they can be.

What made the dx11 run worse and can I do anything about it?

Was my information on dx11 being better for overall system performance in error?