Bernie seems to win every type of online poll, whereas Clinton seems to generally do better in phone based polls (although even that seems to be changing as the latest poll in Iowa shows Sanders ahead by 5 points).
One difference between phone polls and online polls is that online polling is an act of will - people initiate the action on their own. Even if they get an email asking for their response, they choose to respond to the email. Phone polling is more of a passive activity. You answer the phone and respond to questions.
So I wonder if the difference between the results of online and phone polls is indicative of different levels of enthusiasm between Sanders and Clinton supporters. Sanders supporters choose to act in support of their candidate whereas Clinton supporters express support when queried. They don't go out of their way to support Clinton.
Locally, this seems to be the case. Public debate watch parties have Sanders banners, pamphlets, posters decorating the theater and the participants cheer louder when Sanders speaks. The local group of Sanders supporters has an active Facebook page and does roadside rallies. There's nothing like that among local Clinton supporters - the enthusiasm just isn't there.
So MSM, reliant on phone polls, may be doing another "Dewey Beats Truman". They may be missing what's actually going on.