Means this that sigma makes the lenses better for canon ... ?
A bit off-topic.
Tamron & Sigma lenses are expected to perform better on Canon than on Nikon mount, in my opinion.
We have to consider that Nikon crop factor is 1.5, while Canon's one is 1.6 and, therefore, an APS-C lens optically developed in order to get both mounts should have (slightly) less vignetting (and better transmission, maybe?).
If the Sigma lens fits the Nikon mount, it has 13.7% more glass than strictly needed for a Canon APS-C mount.
To put it in other words: full frame lenses produce low vignetting on APS-C bodies, and a Sigma "Nikon Compatible" APS-C lens is slightly more "full-framish" than a Canon APS-C lens.Nevertheless,I will not be surprised if similar results emerge from DXOMark tests on Full frame Nikon vs. Canon bodies with Sigma / Tamron lenses.