Sanders typically outperforms polling. He's much bigger among young voters, most of which aren't accounted for in these polls. Iowa and NH is a statistical tie between Biden and Sanders right now. Those first few primaries could decide the whole thing. Let Sanders win Iowa/NH, and it could be curtains especially when Warren drops out and throws her support Bernie's way.
Bernie does well for young ppl in deep blue states. Not battleground states.
Plus young ppl barely vote.