the Dxo mark score is not the average of the 4 metric scores.
It's another score that takes the sensor noise and dynamic range into account for a specific illumination level. Given that the nikon lenses are tested on the D800, that gives them a noise avantage compensating for lower optical performance
So, as explained in the article, if you want to compare the lenses performances alone, just look at the sharpness, CA, distortion and Tstop scores.
In that case, the Canon are clearly superior as you noticed (especially sharpness and CA). The Nikon lenses are still very good, but the Canon are trully amazing (but a bit more expensive too..)