When selecting some of the biomes under "customized" world type, leaving the interface, then coming back to it, the biome will change 2 points on the slider to the right. However this doesn't happen to the first normal biomes such as forest, desert, swamp, ocean, taiga, river, etc., but once you get passed frozen ocean is when it only occurs. Also- If you go to mesa plateau f, and re enter, The biome will change to "?". I think the "?" means all biomes, because I created a world with it and spawned in Jungle and ice plains.