I believe the problem with iOS (unless my info is outdated) is that it doesn't listen to the scroll event until it's completed. so sticky stuff won't move until the user has finished scrolling and lifted their finger from the screen.
On iPads that docking wouldn't happen until after the scroll had completed instead of at the start of the scroll. I'm not sure if this happens on the latest iOS versions or not though. But it was fun explaining to project managers what was going on during QA.
Analytics shows us that this minor issue with iPads is of little concern. But at least I now have a suggestion for a solution if someone decides it's a big enough deal. Thanks for the tip.
All round, very very irritating.