How does my Casio watch know the altitude in flight? Wrist watches usually use the barometric pressure sensors they have to calculate the altitude. I have also confirmed this from the manual of my watch, which states that:


  The watch displays altitude values based on air pressure readings taken by a built-in pressure sensor.


Fine that's understood and makes sense and has always worked for me wherever I go. 

Question

But then how does it know altitude during flight? The cabin of all commercial airliners are pressurized and hence the air pressure is not changing inside the cabin with the change in altitude (as happens for descent or ascent on ground). I know that even the aircraft itself uses pressure sensors for many calculations but those sensors are mostly usually on the Pitot tube and they sense the real outside pressure and not inside an pressurized cabin

I would also like to mention that yes the altitude reading it takes during a flight is not always close to what the real value is but still it does track the altitude difference when pressure inside the cabin remains same, how so?

Please note

That I didn't include the exact details of my watch because neither am I promoting any special model or a brand nor am I asking specifics of how a watch works, focus is on air pressure inside the cabin, not on watch. If needed I can provide the model and manual reference links.
The pressure in the cabin does not remain the same. It decreases, but much more slowly, in such a manner that when the aircraft is at 37000 feet the pressure in the cockpit is the equivalent of 7000 or 8000 feet. Keeping the cabin pressure at sea level would pose incredible stress on the fuselage, due to huge difference in pressure between the inside and the outside.