Well, this particular pool seems to be for 1 artist thats been doing alot. So i suppose if an artist takes it upon himself/herself to go past 5-6, give them their own pool? i dunno.
That's it exactly. This one is for the girls confessing to the viewer; the other, "Love Confessions in Gensoukyou", is for the girls' response to having someone confess their love to them.
Additionally, the other pool is composed entirely of image-responses (basically, pixiv art memes) under the tag 告白幻想郷, whereas this pool's tag is 東方告白祭り.