As I've said it's a simple issue to understand.
1. He's black, the left has used white-shaming to their advantage for 8 years, and naturally people want to show they aren't racist and will do anything to prove it today. If Obama was white and had the same record, his approval would be in the tank, probably under Bush's. Just look at Jimmy Carter for reference. Wasn't as bad(albeit bad), 1 term, terrible approval.
Or
2. They're polling 80% lean liberal like they were doing in the election.
Or
Combination of both.
Personally, I think it's majority latter, with some option number one mixed inside the cake.
He campaigned for the left, he's been president for 8 years, he got most of his agenda accomplished.
The left lost more gains than any party since the 20's.
High approval my ass.