Revelations of Mr Trump's "appalling" behaviour in the past had also shocked a lot of voters, Mr Muller said.
But the FBI's signals that it was looking further into the Clinton email server issue had also affected polling.
"I think that's reminded people of some of the perceived character flaws, of the Hillary Clinton trustworthiness, and so the gap has narrowed."
Despite this, Mr Muller still believed Mrs Clinton would win, with roughly 300 to 320 electoral votes compared with 230 to 200 for Mr Trump.
"If he wins all of the swing states he can get there. But I don't think that will happen. I think she [Mrs Clinton] is likely to pick up some of those.
"It's so unpredictable. The polls suggest Hillary Clinton has this reasonably comfortably, but that assumes the voter turnout is reflected in the polls."
Mr Muller predicted interesting times ahead no matter which candidate ended up winning the election.
Mr Trump would be unpredictable and Mrs Clinton would face four years of "intense gridlock" if Republicans continued to hold the House.
"America, the great democratic experiment, has reduced themselves to these two choices.
"Independents are despairing."
Mr Muller said what "absolutely astounds" him about American politics was the passion on display.
"In New Zealand, most people have a light interest in politics that becomes a bit more front of mind at election time but it doesn't dominate their everyday lives.
"In America, politics is a major part of daily conversation and most people have passionate political views. They're not only passionate in support of their candidate and party, but passionate in their anger at the other candidate.
"It's a white-hot anger. Normal people are reduced to steaming vernacular."