It's fascinating...at least to me.I had my wife buy one the day I began my quarantine. Should she have been out at the store?...another argument. (She tested negative that same day)
Anyhow, what I found over my quarantine, was that the colder my hands were, the lower my O2 sats read.
And I could get a range of readings depending on which finger I used.
So... my advice is A) Make sure your hands are warm. And B) check more than one finger.