“The big question for this election is this - how many people are going to secretly vote for Trump, but will never admit it in public or to a pollster?”
The Zogby poll is an online poll so it should account for most, if not all of the ‘secret Trump vote’. The other online polls have generally been showing better for Trump than the phone polls, probably for this reason.
Online polls are popularity contests that the most excited people answer.
They don’t sample.