User blog comment:B214/Light Novel Volume 16 spoilers/@comment-9339850-20131031065534/@comment-1359322-20131103034604

@JJJ, that's cause those publishers forced the author to make two SS Volumes under such a short period of time. Volume 13 and 15.

@Crimson, well that's why Rias and Akeno have a landslide advantage over the others. Well i am interested on the dates too but i'm guessing the author didn't include them because there are too many, imagine if one date = one SS, he would need to write about 11 SS in total.