Climbing floors

Hello everyone.

I have a ForeRunner 255 Music.

If I climb up from ground floor to 4th floor, I suppose that I climb 3 floors. No ?
But my watch gives me the result "2 floors climbed".

I have set up the GPS (outside).
And there is no difference...

May I ask another question here or post at a different place ?
If I walk inside a flat, do I have to select / choose "Inside walking" or "walking" please ?
Sometimes, the time resulting to those climbing is "00:00".

What should I do please ?

Thanks in advance for your answer.
Have a nice day !