It is interesting how close the REP result is to the recent Yougov poll is, and how different the Dem side is. R 34-18 IBD, 36-19 YG. D 43-39 IBD, 54-37 YG
The pairs of numbers suggests that YouGov has a smaller undecided pool. On the Dem side IBD has 18% undecided (+O'Malley) and it's 9% in YouGov. It could be that YouGov gets more of the lean establishment to commit than IBD. The undecideds are much higher for Pubs on IBD, too: 15% to 6%. If the same effect is happening on the Pub side it might not be noticed in the top two since neither Trump nor Cruz is establishment.
