actualy, thats exactly what he is doing, otherwise he would mention those 10 specs where most chosen (not always "best", mind you) covenant have under 50%( i would even consider 6 that are under 55%, but lets keep to those 10 for now), which means majority of people playing those specs did not in fact chosen the "best"...
he would also not ignore that highest disparity is for specs where the same covenant is both best aestheticaly fitting and best performance, so we cant realy say what was that choice based on... and thats only considering those two criteria, not to mention there are dozens if not hundreds different criteria people used to choose covenant...