One possibility is that Dave Z knew that he was not a realistic contender on the stage (which, VN prognostications notwithstanding, he wasn't) and thought it would be better to dial it back and cruise in rather than blow a lot of energy for no good reason.

ETA: and by "possibility," I mean "extremely likely." Even on a course like this, Dave Z would be a good deal better than middle of the pack, just not good enough to really threaten for the win.