One factor in reports of 'poorer than expected sales' could be the huge range in Apple's current iPhone line-up, diluting sales for any individual model.
New iPhone purchasers have a wide selection to choose from, and with so many price-points, the newest or 'flag-ship' models may not be as attractive as they once were.
Apple currently sell 8 iPhones, all with between 2 and 5 colour options, and 2 capacities.
X - has 2 colour, 2 capacities
8 - 3 and 2
8+ - 3 and 2
7 - 5 and 2
7+ - 5 and 2
6s - 4 and 2
6s+ - 4 and 2
SE - 4 and 2
That's 8 different iPhones, in a mammoth 60 varieties (or SKUs). Extraordinary (for Apple) ... !!