But my thinking is that if the sources of data are based purely on "units shipped" as per [2], then this relates to both consumer and commercial use. We know that HP, Lenovo and Dell basically dominate commercial sales - thus a drop in consumer PC sales alone could be highly correlated with a drop in Apple sales.
A drop in consumer sales could be due to negative shift in design and direction as many have suggested. But it could also be due to many macro-economic factors that simply do not affect the people here, and are thus hard for us to appreciate - Brexit, no. of mobile devices owned, <insert more speculation here>. As developers or designers, we're not exactly price-sensitive when it comes to buying PCs - so it's likely to be about value proposition and getting exactly what we need. But this cannot be said for everyone.
Speaking for myself, I had the opportunity to buy a Macbook Pro last year, but I chose not to due to value proposition, even though I'm willing to spend the cash. I considered the Lenovo X260, Macbook Pro 13", and Dell XPS13. I will admit a bias towards Thinkpads, and I'm indifferent towards Mac OS X (my only OS preference is "not Windows"). I went with the X260 not because of said biases, but purely because at the same price for the Macbook plus one or two upgrades, I got a maxed out X260 with an incredible battery life, plus left-over $ to buy an SSD for Linux (I've never booted the stock hard drive). Screen resolution was the only compromise. My wife then inherited my 5-year-old X220 and its docking station - still running strong.
BTW - it always gives me a smug smile inside when Apple or Dell markets a 12 hour battery life as though it's miraculous. I had that on my X220 which is now nearing 5 years old (oh - and I could buy a new battery for it as recently as November 2016). Close to 20 hours with the battery slice.
[1] http://www.idc.com/getdoc.jsp?containerId=prEMEA42255917 [2] http://www.gartner.com/newsroom/id/3474218