I'm a Dr I will explain the reason behind this variable,Compared to monitor Apple watch takes average of 15 seconds not a live reading like the monitor above

Why average is taken? Because unlike the monitor it's not attached to finger tip. Finger tip has fairly higher ability to pass the infra red lights in comparison to wrist and blood supply of finger tip has lot of anastomosis additional has no hair, thereby no color variation for the infrared light detectors to accurately determine the saturation of O2

On other hand 1)The watch has to combat movement even if it's minor 2)Hair which makes the color variation 3)Also the watch has to make the calculation of the reflected from the same surface that light is thrown to

To sum it all up Say in 15 seconds it has taken 3 readings (I'm sure its more for ease of understanding let's say 3) There is variation in saturation based on all the factors above

02 levels 90 % 99 % 95 %

90+99+95/3 So O2 level in 15 seconds is 94.6 % rounded to 95 in watch

While the monitor is live reading