That is the conclusion that I am coming to and IMO a serious weakness in the Apple Watch and means that apps like NapBot that claim to help diagnose sleep apnea are a waste of money and time.
On speaking to Apple support earlier today, the guy mentioned that the number of readings he got during the night and day (when background measurement was on) varied but he couldn't explain why.
Well..what I think happens is that it tried to take the measurement but if the watch happens at that particular time to not be oriented correctly then it will abort the measurement.
TBH I have come to the conclusion that NONE of these wearables, by any manufacturer, will provide adequate blood oxygen level readings.