Notice how in every single volume cover, there's multiple girls? EXCEPT for 4, 7, and 10, where Shinobu is the face of the cover. This represents how being best girl means you'll stand alone at the top.
lots of people have noticed it's the same pattern every time:
rikka cover has everyone,
nene cover only doesn't have shinobu,
iko cover only has rikka with her.
so there is definitely something to it, but it's not confirmed what yet.
I think most likely it's related to how involved their profession will be with the others, like recent chapters show that:
shinobu will be the last person who will accomplish her dreams
chronologically (because she needs to take a college course first , but the others don't)
iko has gotten rikka to write her a song in chapter 70s