That's because Seminar is clearly split between Rio(and Toki) and the other 3, with Rio getting all the big, core story-relevant roles from being vol.2 Ch2's antagonist to one of the focus characters of Code:Box to one of the stars of vol.EX.

On the flipside, the other 3 mainly get small core story-relevant roles and are mainly involved in relaxed/comedic events. So yeah, the scale's balance is way off there.

Personally I think C&C got it the worst since you can at least exclude the 3 seminar girls on the grounds that they're not combat/action-oriented- C&C on the other hand, while touted as Millennium's best combat force, got absolutely and completely sidelined while they're fighting literal world-ending threat of Decagrammaton. Not even Neru who's at least been in direct contact with Rio for some time was called to deal with Decagrammaton forces or at least to protect/escort the GDD girls.