There's actually nothing wrong with DxO's scores.
Does M9 have huge dynamic range and color depth? No, it's fairly average. Is M9's sensor good in low light? No, it's a CCD, and currently CCDs are more noisy than CMOS sensors at this size and pixel pitch.
So what's actually exceptional about M9's sensor? First, its custom microlens array, which reduces vignetting - but DxO doesn't test sensor-lens combos; second, its resolution due to high megapixel count and the lack of an AA filter - but DxO normalizes the scores to preset resolution; so all of this doesn't affect DxO scores.