I really don't understand this line of reasoning.
In the months leading up to the release of the iPhone 6/6S wasn't the common belief that there was huge pent up demand for iPhones with larger screens?
Which proved to be the case when the 6 came out, and continued to have record breaking sales right through to this equivalent quarter last year.
So now, a year later, it shouldn't really be that surprising or shocking that the figures haven't matched that spike from a year ago, because this year didn't have the same pent up demand as last year.
I'm no analyst or anything, so I could be barking up the wrong tree.