One could conclude that Central, Southwestern, and Western shooters are better shooters. Thus, if you can't maintain the higher average, you need to step up to be more competetive.
Or one could conclude that there are more sandbaggers in the Eastern and Southern zones, and the lower averages required to recieve a reduction are to make it harder for the sandbaggers to get a reduction.
Or there is also the possibility that due to the differing number of shooters, targets thrown, scores being shot, and weather conditions between the zones, the diffferent break points were established so that similar rates of reduction offers were made between the different zones.
And there is the possibility that it doesn't matter one bit that the break points differ by one percent between zones, since finding a yardage where you can maintain an average of either 89 or 90 doesn't make you any more competetive.