1) poll taken too early (e.g., before any Dem indictments occur, before the Wall is more significant, etc)
2) Biden and the others have not had any exposure to voters for what they really are
3) not clear if registered or likely voters or what
I took a senior level statistics class from one of the professors who worked for Richard Wirthlin, Ronald Reagan's pollster.
One of the more memorable lectures was on games pollsters play. There are a lot of them, including those in play here. A good pollster always asks their client if they want an accurate poll or one which sounds accurate and will confirm what they are selling. If the later, then they use a lot of weasel words in the poll and the design of questions