Low temps can be a b*tch. Height of the thermometer above ground and other issues. Some places used rooftops as well. That's a big problem with the old Seattle city records. They were done on top of an office building. No way that is really representative of temps on the ground.
I've wondered about that Rogers Pass, MT -70F record before. Apparently there's a Peter Sinks-like geographical bowl near the pass where that reading happened, I did a deep dive on the circumstances in which that record was taken.
There's an old thread on here about it:
They used mercury thermometers which are extremely accurate if well made and calibrated. Also...if I had been keeping records back then I would have been well aware of exposure problems. It's just a common sense thing.