Google reports that the usage of Jelly Bean is 37.9%, which is finally taking the first place from Gingerbread with 34.1%. However, Google recently changed their data collection of Android fragmentation:
Beginning in April, 2013, these charts are now built using data collected from each device when the user visits the Google Play Store. Previously, the data was collected when the device simply checked-in to Google servers. We believe the new data more accurately reflects those users who are most engaged in the Android and Google Play ecosystem.
By only looking at the Play Store Google does indeed measure the most active most engaged users. I am afraid that it is the less active users that are on the old OS’es and as such are a big risk. The better way to measure these users is by looking at the email logins. And still then you are missing out on users. I am afraid there is no definitive answer, but changing the method and then saying that it got better is definitely the wrong way to do it.