Conversations with Nitro suggest that is based on level rather than quality. A higher level generally means a higher quality but it's not really the same thing. I think it may have been introduced to control the possibility of a lower level beating a higher level. I know some players had been bitching about this. At the time of introduction it certainly became very difficult to beat a team even a single level above. That's changed since which may have been due to them adjusting the effect or even removing the factor altogether.