Some people forget how hard it is to defeat an incumbent President. The last two incumbent defeats (1992 and 1980), there was a huge 3rd party factor that, at the very least, complicated the picture. 1976 Ford almost pulled off re-election despite the baggage of Nixon pardon.

This might explain some of the "surprising" Trump strength, BTW. There's a strong tendency in some demos to re-elect the incumbent (just as there is a strong tendency in others to vote against the incumbent every time). Might explain Obama's 2012 strength (and W's 2004) strength.

The President has enormous power to wield to please certain demos, esp. ones with lots of skeptics in the first go-round. Miami Cubans are a perfect example here. They had serious doubts about Trump in 2016. For whatever reason, those doubts were assuaged and they came back big.

Sometimes there's a "disbelief" factor that keeps people home the first go-round. After one term, voting for them seems "real." African Americans in the Mississippi Delta going stronger for Obama in 2012 than 2008, for example.

But look at the increased margins for Nixon '72, Reagan '84 and Clinton '96 v. their first run. Sometimes people take time to adjust and accept. Obama's case there was a big fall-off BUT his incumbency resiliency sustained him...and beat the polls.