This election made me just not care about approval ratings or polls. They are totally just meaningless statistics for me now. Trump showed they don't matter
Actually they were pretty accurate in '16, nationally and stade-wide. All RB states were within the margin of error except for WI, while Trump has slightly ahead in NV.
To the OP's question: Internals are often biased or under/overpoll certain voter groups to get a better results for the candidates themselves to gain momentum ("see, we're winning in the polls").
A few, especially MO, IA, WI, MN, were off by more than 3.5 points, when looking at the average of the polls. Trump under polled in each of those states. Why that is, I don't know.
Most polls in 2016 simply didn't weight by education. This had a big effect in those states with a lot of non-college whites, and it was compounded by the fact college educated whites are simply more likely to respond to polls than non-college whites do.
