Interaction between zooming and scrolling gestures in a UIScrollView?

I have a UIScrollView taking up about the lower third of the screen on an iPhone 6s. The content view is an UIImageView displaying a typical iPhone camera image 2100x1575. When the app first puts an image in the UIImageView/UIScrollView, it is well zoomed in because the scroll view is only about 400x400. At this point single-finger scroll gestures have no effect. If I do a pinch gesture, the zoom effect "wakes up" and I can manipulate the image, but in a way that confuses me. How far I can scroll in the image seems to be affected by the zoom level; surprisingly (to me) I can roam the zoomed-in image over a wider range than the zoomed-out image.

I tried to make sense of what's going on by putting print statements in the scrollViewDidZoom and scrollViewDidScroll delegate methods, but when I do a pinch gesture, I get both zoom AND scroll callbacks from a "purely" pinch gestures.

Where can I find an explanation that deconvolves the behavior of the scroll view?
Interaction between zooming and scrolling gestures in a UIScrollView?
 
 
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