Default behavior is for each map to color the map with histogram equalization, so 10% of the hexes fall into each color category. This is what you get when the Free map scale button is selected. Select Retain map scale and then all subsequent maps will retain the same coloring scale until you reselect "Free map scale".
You might want to retain the map scale to compare biomass levels in different states. With the "Free map scale" option, colors are not comparable across separate maps. However, it might not make sense to use a map scale developed for a county map and then retain it for a hex map.