I don't really think it's a feature but I know what you mean. It seems the only designation here is "Bug" and that's what I think comes closest to how I would describe this issue.
Superflat is different as it actually allows you more customization. Amplified and Larger Biomes can be recreated in the Customized settings so their being world types is redundant. TheMogMiner confirmed this.
I don't really think it's a feature but I know what you mean. It seems the only designation here is "Bug" and that's what I think comes closest to how I would describe this issue.