There's a whole range of thing that can affect pulse oximeter reading, cold, poor circulation, bright lights, heart rate, (very fast or slow and peripheral circulation may be affected)
They're an interesting test in that if you see someone and expect their sa02 to be normal but find its low you tend to explain it away with one of the above reasons
If they're breathless or unwell you take low Sao2 seriously but only as part of the bigger picture, as if they are breathless and unwell with a normal Sa02 you still need to keep looking for the cause
Many people with chest disease such as COPD will not have had normal sa02 in years.
Other observations are needed to understand what going on as Sa02 falls you would expect respiratory rate to rise, fever would come with infection
Every now and again our local nursing home re-discovers their pulse oximeter, this is followed by calls about Fred and his low Sa02, Fred is 96 years old and knackered, his Sa02 hasn't topped 94 in 20 years. (most nursing homes have 20 Fred's or Freda's)
I discovered that night shifts have a marked affect on my SA02, not sure why.
Sao2 is a bit like temperature, a child with a temp of 38 may be very unwell, but less likely if they're tearing around the waiting room. You need to use the numbers as part of the bigger picture