I wish I had a snarky answer, but the real fact is that polling has become less and less reliable. Cell phones making it harder to reach people and people don't pick up or return calls from numbers they don't know, especially younger people. Also, like so much else since Citizens United passed, everything is a commodity, including polling -- a lot of polls have popped up over the last few years that lean right, which floods the field and skews the averages.
Also, look at the demographics within some of those polls. When a poll comes out that gives Trump a 3-point lead and he's only trailing by 3 points among women, be skeptical, because virtually ever poll, including the right-leaning ones, have Harris well ahead with that group.
I give the polling a little attention, but the fundraising, early ballot requests by party affiliation, general energy of a candidate's support all are things that might be better indicators. I think also watch the candidates themselves to see if they seem to be panicking; if a campaign does some extreme about-face or does something that smells really desperate, that might indicate their feeling and internal polls tell them they're in trouble.