Really? Why is Rasmussen doing that? Are you sure he is using D+6?
There is no place where Rasmussen provides his sample. The D+6 is a tweet from NumbersMuncher who has done great work analyzing, crunching polls.
Rasmussen provides a weekly crosstab and it is within this crosstab that the sample has shown to be 39/36/25.
The D+6 is simply a guesstimate. But if you do the math, it would be impossible for Romney to be up only 3 while leading Indies by 17 and scoring 13% of Dems (with a D+3 sample).