Naturally there's a variation depending on the Operating System, but Symbian is relegated out of the results with it being an American company surveying American users. The headline has a value of 582 MB belonging to Android, down to 127 MB for RIM users. Windows Phone sits somewhere in the middle on 327 MB.
There's a danger in reading too much into these numbers, and trying to answer why a certain platform is using more or less data than another. It's likely that the survey just isn't detailed enough to get a decent level of accuracy, and the market segmentation between a 50 year old CEO and his Blackberry, and a 16 year old iOS user is going to add another distortion, but overall it gives us a number to work on.
I wonder if the networks who set the data caps will pay attention?