Lots and lots of users don't upgrade. I can't find it right now, but one iOS dev did a story on fragmentation in that camp based upon their own metrics, and it was scary. Perhaps this will resolve if they start doing OTA updates.
User "error" for sure, just putting that out there. Just because Apple makes updates available doesn't mean that most users have installed them.
I'm not sure if the bump numbers are broadly relevant: who is the average user who will be running bump? It isn't a general iPhone user, but instead is a technologically savvy individual usually in a place like San Francisco. There is no relevance there to the broader market.
User "error" for sure, just putting that out there. Just because Apple makes updates available doesn't mean that most users have installed them.