Acchan clearly introduces them as “haro-puro kids soshite berryz koubou"
Do you mean just because she’s stood back there as if together with all the BK girls?
I think it’s because Aya Matsuura opened the concert with six ‘kids’ as backing dancers, and they chose the non-berryz-koubou members, but minus Yajima, to do that. That’s why they were separated and the other six have got an extra part to their costume at this stage?
They must just have just thought six provided the right visual balance, and one of them had to miss out.
Later in the concert, Maki Gotou has four ‘non-berryz koubou’ with her in her number, and that time Yajima did make the cut!