It's interesting to me that the GCB is D+6, but in the two questions of "who do you want to win the "Senate/House?" it's D+2 and D+1. Assuming that "no preference" in those two questions correlates roughly to "not sure" and "I would not vote" in the GCB question, the biggest difference seems to be the 13% of voters who favor divided government, who presumably tend to favor Democrats in the GCB.
Could be a theme worth watching, since in general I would assume pro-divided government voters would tend to vote against the president's party in a mid-term. Another plausible read of this discrepancy is that Democrats have stronger candidates.
