You’re correct that P20-2 to P21-1 is a useless measurement. It’s not even reading across the same switch, and it wouldn’t really tell you anything.
Let’s break it down this way.
Let’s say you turn on broil and then put one probe on p20-2 and the other on p1-N. If the switch from p20-1 to p20-2 has closed properly, what is your expected voltage reading?
And if that switch hasn’t closed, then what would be your expected voltage reading in that case?