"based on the number of Android devices that have accessed Google Play within a 14-day period ending on the data collection date " (September 4, 2012)
14-day period? That's a pretty decent way of making sure that the people who have had their phones for a long time (like this guy here with the Droid X) are underrepresented.
They're getting data from devices which accessed the play store, because that's all the data that google has.
Otherwise, how would it know to pop up a notification that app updates are available?
This statistic is published for developers to get an idea of which OS versions they should be supporting based off market share. If you aren't opening the play store to buy apps in the 14-day period, then these developers probably aren't looking to sell to you.
Besides, I doubt most people know how to turn off background activities.
Seems somewhat plausible to me.