When switching back and forth between the Customized World GUI and the Presets page, some scroll bars will be set to a near number. This has the minor implication of some settings being rounded to an even number. But it is especially an issue in the Biome selector. Any biome with a value of 8 or greater will be rounded to the next highest even number when switching to these tabs. So 8 becomes 10 becomes 12 becomes 14 and so on.