This weeks polls internals...
WaPo D+8
Fox D+9
NBC D+12
Reuters D+15
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I think I get it. They are assuming that the dems will be so enthusiastic that 18.9% more dems will show up. Wow!
Firstly Dems outnumber Republicans. Also, the over sampling is representative of the lead. The candidate with more support will reflect on higher party identification. For the most part polls try to be accurate and they publish all of the results. This poll might be an outlier although the trend on RCP is confirmed.