Many of the RV adults of 2016 are not registered voters in 2014.
Turnout in open-seat Presidential years is usually high, and it could be even higher in a year in which one of the Parties is extremely unpopular.
I really doubt the figure is "many." The number of voters who aren't registered during the midterms, know it, and re-register before the next Presidential isn't that high. You're limiting your population to super-sporadic voters (rarely turn out), the newly-18 (not that many people), and those who move and are aware their voter registration lapsed (not a ton of people).
Just looking at the Washington State database, under 10% of the 2012 electorate first registered in 2011 or 2012. The rest were ongoing registrations, or inactive registrations that saw address update. It may be a higher figure in poll-voting states, but still, it's not a high enough figure that it's going to make a big difference in opinion polls. There aren't very many voters who will admit to not being registered in 2014, but will actually vote in 2016.
Let me put it this way: The 2016 electorate will be a lot closer to currently registered voters (that is, ones registered today) than all adults. In fact, the number of now-registered voters who won't participate is
much higher than the number of currently-unregistered adults who will.
So, the 2014 electorate will not be anything like "all adults." It will even be smaller than "registered voters" (and by that mean, I mean voters registered today). It will be somewhere between "registered voters" and "2014 voters." In fact, looking at the Washington database, 72% of 2012 voters also voted in-state in 2010. Presidential voters generally are Midterm voters, and among those who aren't, most were registered during the Midterms -- they just didn't show up. And among those not registered during the Midterms, the vast majority won't show up for the Presidential, either.
tl;dr: The 2016 electorate will look a lot more like the 2014 electorate than "all adults." It may look more like the 2014 electorate than "registered voters." Anyone who tells you otherwise can't do math, or is a delusionally optimistic Democrat.
