Thanks for your interest in DxOMark.
We rank lenses with two main scoring systems: DxOMark score and Sharpness score. For both, we compute each score for each couple focal/aperture, take the best for each score and average each focal results (see our documentation http://www.dxomark.com/index.php/About/Lens-scores
As the use case for DxOMark is more challenging the lens with wider aperture mounted on camera with a good performance in lowlight will score higher.
For sharpness, we only take the best aperture for each focal.
So the high apertures are mostly not taken into account for the two scores as these apertures do not provide the best score (neither DxOmark score nor Sharpness score) because of the diffraction impact.
In the case you report, the Samyang achieves a higher DxOMark score (because of its wider aperture) but gets the same sharpness score (their best sharpness is very close).
I hope you will find these few lines useful.
The DxOMark team