Why is he doing this? The D+6 refers to the expected turnout, correct? If so then this doesn’t square with PEW RESEARCH which has Republican enthusiasm at a RECORD HIGH.
It's a messy number. Some people register with one party, change their loyalties, then vote the other party, but never change registration.
Party ID is according to what people SAY at the time of the call. It may or may not be an accurate number. In open primary states, there is no incentive to keep party registration accurate. You have self-identified Democrats who haven't voted for a Democrat in 20 years, so these "high numbers" for "Democrat" voters may in fact be accurate.
He says that he is not manipulating that number one way or the other. He says they simply make the contacts and, based on the answers to questions about past voting history, intention to vote and enthusiasm, he classifies them as "likely voters" or not. Then he asks about party ID. Specifically, he asks "as of today, do you consider yourself R, D or I." If his contacts reveal a party advantage in the likely voter model, it's simply the fact that that is what he is seeing according to his model.