And Biden won by 10 pts when being favored by 17. Harris plus 1 means she's down.
Every election, pollsters modify their methodology based on the previous election. I'm sure there will be mistakes, but they'll be different mistakes.
Last time, Biden's polling tracked his numbers in the election. Trump's improved. Your imagined predicted margin of 17 didn't exist. Biden was predicted around 51 and a bit. And got that in the election. Trump was predicted to get 43 and a bit and got 46 and a bit. Easy Call #1: Harris will win the popular vote. Who knows who will win the Electoral College? Easy Call #2: if Trump doesn't win the Electoral College outright there will be shenanigans aimed to slow walk the election to the House.