Thanks for your interest in DxOMark.
Between design and real lenses there is always differences !
For this case, the first remarks would be that the 2 sensors have very different optical measurements. On one side the Canon EOS 5D Mark III is very efficient, while the D800 is finally slightly disappointing when considering resolving power.
The second remark is that vertical sharpness is slightly lower measured on the Nikon mount.
The DxOMark team