Real Clear Politics uses selected (i.e. "live interview" )polls for its nationwide polling and for its "Poll of Polls" averaging.
For the state polls RCP uses all polls by whatever mechanism they use, possibly because state polls are less frequent than nationwide polls and if they didn't change their mechanism they would have less to report.
The Huffington Post pollster uses all polling companies (which they tout as an advantage) for both nationwide and state polling. I have noticed that, in the end there is very little difference between the two outfits aggregate numbers-Huffington Post seems to give Trump a few more point leads over his respective rivals but not hugely so.
What seems very odd with the crucial Iowa caucus coming up is that RCP has left out the Emerson College poll which shows Donald Trump up by ten points over Senator Cruz 33% to 23%. This is Trump's second largest lead over Cruz since November 2015 and is just one point behind the CNN poll which logic seems to confirm both.
Further it includes polling taken after Trump was endorsed by Sarah Palin
The Emerson College poll is substantial, 882 respondents, and the fact that RCP includes Gravis Polling which is also a random automated phone method is surprising and challenging-there appears no reason why the Emerson College Poll should have been left out.
Here is the Huffington Post list of recent Iowa polls and the trend line which clearly shows Trump ascending and Cruz descending (Red and Black lines).
And here is the RCP list of polls without Emerson College