DxOMark has run the venerable Nikon D800E through the ringer and it has come out on the very top of the digital camera heap. This is quite odd considering that it's the same camera as the D800, it only has one extra layer over the sensor.
The D800E is an odd sensor. One would expect a company to simply remove the AA filter if it wants to eliminate the AA effect. But apparently this would have been difficult, requiring two distinct manufacturing lines. Nikon's solution is to have a line that makes the standard sensor and another line that makes the AA filter sandwich. The sandwich has three parts, two of which are diffractive. One filter diffracts horizontally, the other vertically. The last step in that sandwich is switched from the vertical filter to another horizontal filter that acts like a de-diffraction filter.
With all of that optical jiggery going on, one would expect, if anything, a slight drop in ISO performance since no filter, no matter how perfect, will not transmit 100% of received light. Somehow, the D800E actually exceeds the D800. The only thing that I can think is that the act of diffracting a a serving of light over a larger area results in more light being lost to areas on the sensor that cannot collect photons, perhaps even off the edge of the sensor.
Also, it could just be variations between camera examples.