That's true.
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.
The A10X is actually the smallest die size in an iPad SOC for many years, thanks to the 10nm node. That means it's the cheapest to produce. I don't see Apple willingly cutting into their profit margin by building a larger A11X chip until it's absolutely necessary to maintain sales volume of iPads.
When looking at the longevity of recent models I think the lifespan of iPad models is getting slightly longer than 12 months:
iPad Air 2 (Oct 14 - Mar 17)
iPad Pro 12.9" (Oct 15 - Jun 17)
iPad Pro 9.7" (Mar 16 - Jun 17)