More like Blackberry "undertook" WP in US, since its decline rate has been much faster than WP's growth rate in US.

Sites like TheVerge [1] said exactly the same thing when it happened worlwide, that WP overtakes BB, even though this is how the market share charts were looking like back then:

http://static3.businessinsider.com/image/5192a95969bedd70220...

As you can see WP's growth was snail pace back then, yet it "overtook" Blackberry. We're seeing the same kind of misleading headlines now, I guess because they get more hits and shares from Microsoft fans. Journalism at its finest.

[1] - http://www.theverge.com/2013/5/16/4336298/windows-phone-over...