In the population of districts case, there was an objective measurement: the difference in population of two districts. Then, the defendents made a subjective

In the population of districts case, there was an objective measurement: the difference in population of two districts. Then, the defendents made a subjective argument that they were "close enough". However, the Supreme Court doesn't like to replace an objective measurement (population) with a subjective one ("close enough"ness). So, in this case we have an objective measurement -- voter efficiency -- and a subjective one -- "what do other maps look like". If we can sometimes reject the objective measurement in favor of the subjective one, then it's not really ready to be applied as the standard by our highest court.