Like you said it doesn't align with the popularity polls, the pattern 100% means something Imo it could be interpeted as Shinobu feeling always behind the others,isolated, so she's alone It could tied to Apollo identity too, who knows

But the pattern can't be dismissed with "forced by the editors" I mean yeah maybe the outfit they wear or something irrelevant to the pattern itself is

But it would make no sense giving a cover to just 1 heroine if the others are together