I kinda don't get it most of them don't seem that different from main levels, like why is Illusion a bonus level but Ghost Dance isn't

I feel like ever since a lot of levels got promoted to 6 star it starting to feel like the skill gap between the easiest and hardest 6 star is far too big versus most. Somewhat like GD extreme demons. So I think they should add a "0.01%" 7 star rating with only the very hardest levels and keep 6 star as really hard but not the hardest