Didn't I say that there was the album released in July?
Don't say to me that releasing the album isn't counted in this case. It's similar to releasing the single, though the album contains many more songs. Furthermore, the album is worth being promoted too, not just something trashy.
About December release, it's not about another H!P group releasing single in that month, it's just that I think it's a very bad month to release the single. The last month of the year is not an ideal month to release new single (just in common, not anything related to logic).