Really the X chips seem to have matured from being an iPhone chip running at a higher clock speed with better graphics bolted on to being almost a half step between iPhone chipset generations. Case in point the A10X which shares it’s core design with the A10, but built on the same 10nm architecture as the A11. It also ups the core count (something that started with the A8X) but interestingly to a 3 & 3 design, not the 2 & 4 of the A11. I’d guess that’s due to the greater emphasis on power where they have more room for battery and heat dissipation in the bigger iPads and efficiency in the smaller iPhones?