I like Britney's music for the most part, but the over the top reactions and screams of "GODNEY" to anything she does when she doesn't have half the talent of mortally wounded goat makes me rebuke her a bit.
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.