polls rely on responses from individuals so you can lie when asked.  There is also something called the margin of error, which needs to be known so that you know how likely the result is.  Since most US polls do not include a margin of error, it is easy to get mislead.  Also in the US, the polls were right about the popular vote.  The electoral college is a different matter.  The smaller the poll (by state) the wider the margin of error.  Given how close the votes were in Michigan, Wisconsin and Pennsylvania, someone without an agenda like you would see that the polls were not terribly far off,