They are a professional, non-partisan organization with a top-notch reputation. They have a lot to lose if they get caught fudging the numbers. What they are actually doing is recognizing a new phenomenon in American elections, it is called the Banana Republic Vote.
It is no longer sufficient to forecast the voter turnout; how many voters from each party. In this 21st century it is necessary also to forecast the number of votes from each party that will be counted. So even though a higher percentage of Democrats will turn out to vote, Gallup is projecting that not all of their votes will be counted. So give the guys a break, they are setting the computational model for this and future elections.