Polling is a trick in that it is not only a challenge to accurately measure people's opinions at that time, but to properly figure out a week later when the results are released if the opinions were predictive or reactive.
I sure haven't figured it out. All I know is that there IS a lag of some sort. Let's see in a week where the two are.
Last 7 polls from RCP:
Poll Date Cruz Trump Rubio Carson Bush Paul Fiorina Christie Huckabee Kasich Santorum Pataki Graham Spread
RCP Average 11/28 - 12/13 26.4 25.4 12.9 11.9 5.6 3.3 2.6 2.1 2.0 1.6 0.7 0.2 0.1 Cruz +1.0
PPP (D) 12/10 - 12/13 25 28 14 10 7 2 3 3 3 2 1 0 1 Trump +3
DMR/Bloomberg 12/7 - 12/10 31 21 10 13 6 3 1 3 3 2 1 0 0 Cruz +10
Quinnipiac 12/4 - 12/13 27 28 14 10 5 4 3 3 1 1 0 0 0 Trump +1
FOX News 12/7 - 12/10 28 26 13 10 5 5 2 2 1 1 0 1 0 Cruz +2
Loras College 12/7 - 12/10 30 23 11 11 6 2 3 0 2 1 1 — 0 Cruz +7
Monmouth 12/3 - 12/6 24 19 17 13 6 4 3 2 2 3 1 0 0 Cruz +5
CNN/ORC 11/28 - 12/6 20 33 11 16 4 3 3 2 2 1 1 0 0 Trump +13
www.realclearpolitics.com/epolls/2016/president/ia/iowa_republican_presidential_caucus-3194.html
Cruz is leading in 4 polls, Trump in 3. And Cruz is leading in the RCP average by 26.4 to 25.4