Void is a world generation modifier, not a biome. Void means you get no world generation(at least, no stone, ores, or dirt, and anything which needs some of these things to generate), but without a Biome for your Single Biome symbol to pair with, you actually ended up with no used biome controller due to the clear modifiers symbol if I understand correctly. This means you were lacking Vital Symbols(the biome controller), and it had to generate symbols you lacked(the biome controller, possibly biomes).