Jelly Bean's install base got a huge boost in the past 30 days, but it still only powers less than 7-percent of all Android devices. Today's version number release from Google sees Jelly Bean with a 4-percent jump from November, while Ice Cream Sandwich sees a smaller increase of 1.8-percent with a total of 27.5-percent of all devices sampled. We see Gingerbread still holds the lion's share (and likely will for a while), and Jelly Bean growing faster then ICS. This is because many of the phones running Gingerbread aren't going to be supported with an update, and phones that shipped with ICS are getting updated to Jelly Bean.
The sampling process is pretty simple, just keep track of the version on every phone that visits Goggle Play in a 30-day period. This means devices that don't have Google Play services aren't counted, and devices that do have Google Play but weren't designed to are being counted. It's a bit of a mess, but one that we should be used to by now. Next year, ICS will be dominant while we look at the small number of devices running Android 5.0. We don't have to like it (especially when it's one of our phones a version or two behind) but there's not much that can be done about it. We expect a similar picture next month.
Source: Android Developers Dashboard