You're missing a lot of context. You're examining the contents of the effect, without looking at the cause.
The DNC rigged the primaries before any ballots were cast. They conspired against Bernie Sanders to have Hillary as the nominee and worked to sabotage Bernie's run for presidency. Talking about exit poll statistics doesn't really mean much. The corruption was already well underway.
https://www.politico.com/magazine/story/2017/11/02/clinton-brazile-hacks-2016-215774/