That article is a speculation, the number 400 millions was taken out of the article author's ass.
The figure derived from differencing two other numbers may or may not be correct but it has a non anal origin.
The author's assertion came out from his ass: he concluded that "more than a billion users" in MicroSoft's presentation means "exactly about 1 billion users".
No, 1.5 billions is "more than a billion users" as well.
The author is a speculating bitch, not a journalist.
Have you read the HN guidelines?