I don't think they based it on when it reached the mark. It's a matter of having 3 albums released in a row within a decade that sold 4+ million.
I think a better sounding record would be "there debut albums in a row that reached 4m+ within a decade" (as in 10 years, as opposed to 90's & 00's etc). Well, that way she ties Britney & Shania... but I guess Billboard really wanted to make this into some sort of new record.
"'I Will Be' is a song I have known for a while… From the outside looking in I know that everyone was fighting for Avril to put this on her record but for reasons I will not disclose, Avril, did not... I remember Luke playing this song for me months ago and I was at the point of tears knowing I heard a song that would be the hugest thing ever… I mean, people, be prepared to hear this song at EVERY WEDDING for a LONG time."