Has Katy Perry ever encountered an awards show that she wouldn't attend? She'd probably perform her new single at the Employee of the Month ceremony for her local Wal-Mart if it would help her reach #1.
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.