The 18% number is based on the U.S. market only. Sort of like iPhone SE, it sells because of price. The mini receives a $100-125 price cut every other month from Walmart and Best Buy.
There may be a huge difference between a phablet and iPad mini, but most users don't carry around their tablet.
Tablets are being used most often at home for relaxation. That's the conclusion drawn from the research data referenced in this Adweek article from 2015. Tablets are used outside of the home less than 15% of the time. Given the "stay at home" role, the portability aspect of an iPad mini has little value. A few posters have emphasized the "pocketability" benefits of iPad mini, but in reality few people do that.
Given this information, we see Apple's focus on 9.7, 10.5, and 12.9-inch tablets.
All fair points of course. I'm sure for those who commute and/or travel a lot it's still easier to pocket a phablet than an ipad mini, although there is a definite loss of functionality when trading off to a smaller screen. I've always loved the smaller tablets because they can fit into a back pocket, suit pocket, scrubs pocket, purse, etc all in situations which free up your hands. Unless you have a bag a larger tablet is going to take up one of your hands, which can be a major inconvenience.
For home use I'd rather have the largest tablet I can find. My surface pro is actually too small and I'd rather have something larger, probably 15" or so as long as they could keep it light and keep the battery life decent. For me a 10.5" ipad is tiny, much too small to be enjoyed if I'm in the comfort of my own home.
It makes you wonder also about the difference in profitability. Of course Apple would rather sell a $999 phone versus a $399 tablet. They would rather sell you BOTH, but since that's not working they seem to be falling back on selling the more profitable one, and pushing possible ipad mini consumers to buy a phablet instead. With the 10" and above you almost have no choice as you truly cannot compare any phone to a 10" screen with a keyboard case.