A10X is already on 10nm manufacturing node vs A10's 16nm node. So in terms of power efficiency it's more similar to the A11 than the A10.
iPhones are simply higher priority for Apple as they generate much more revenue.
That's true.
You'll get the A11X in Spring/Summer 2018 and then it'll be beaten once more by the next iPhone's A12 [insert catchy subtitle] in September
Have a look at previous releases:
iPhone 4 - A5 - Sept. 2011
iPad 2 - A5 -
Mar. 2011
iPad 3 - A5X - Mar. 2012
iPhone 5 - A6 - Sept. 2012
iPad 4 - A6X - Oct. 2012
iPhone 5S - A7 - Sept. 2013
iPad Air - A7 - Oct. 2013
iPhone 6 - A8 - Sept. 2014
iPad Air 2 - A8X - Oct. 2014
iPhone 6S - A9 - Sept. 2015
iPad Pro - A9X - Nov. 2015
iPhone 7 - A10 - Sept. 2016
iPad "2017" - A9 - Mar. 2017
iPad Pro 2 - A10X - June 2017
iPhone 8 - A11 - Sept. 2017
Apple have had a pretty consistent schedule of releasing iPhone's with their new chip followed by iPad's with "X" variant chips a month later. This is true for all except the introduction of the iPad 2, iPad 3 and iPad Air.
It's quite possible that we'll see Apple:
a) Quietly update the iPad Pro with an A11X after the iPhone X release or
b) Prolong iPad Pro 2 life cycle until October 2018, potentially skipping A11X and going to A12X (or even A12 like they did with the iPad Air's A9) or
c) Stick to a yearly update and release new iPad Pro's with A11X in June.
I suppose it all really depends on how well they sell and how Apple want to position their Pro line.