So, the 538 polls only got it half right.

[TwilightZone]

4. The numbers don't add up to 100%. The rest (other than third-party) has to be undecideds by default.

Most polls showed Stein at under 1% and RFK under 0.5%. The total of 1% they received is about what was forecast in the polls.

The rest has to be undecideds.

Math:

100 - 1 for Stein/RFK Jr. = 99.

99 - 48 (Harris) - 46.8 (Trump) = 4.2.

Using my simplistic assumption of 50/50;

4.2/2 = 2.1. Add to each side for 99% + 1% Stein/RFK = 100%

"Undecided voters are basically not participating in the poll"

Choosing "undecided" is participating in the poll. If undecided wasn't an option, every poll would be 99% for Trump/Harris because of the 1% for TPs.

If it's not undecideds, what is it? 4.2% is missing and it's not third-party voters.