I think in addition to the obsessed wota buyers there are more casual Kirarin*Revolution watching buyers, thus extending the sales over a longer period. The single is also cleverly marketed. There is the tie-in with the anime, Koharu's appearances at Oha Star (including a mini drama about Kirari wanting to get a CD release) and a karaoke toy thing featuring the song. It all helps and it shows who the potential buyers of today are: kids.