Today I did a walk up a very steep track on my local mountain. I started at elevation 220m and walked up to 400m. Garmin says that 3m gain is supposed to equal a floor climbed. So given I did at least 180m up and then the same down again I should have received credit for 180/3 for the ascent (60) and the same again for the descent, which is 120 floors. My watch said I had done 2 floors before I started this walk. After completing the walk it now shows only 10 floors completed, but it should be about 122.
The other time I notice the floors are always wrong is when I take the stairs in my office building. We have a fire rated stairwell, which may be the problem - there is strong air suction due to a constant fan inside the stairwell. I regularly do from 6 to 12 flights of the stairs each day and it never registers any of them, not even once has it ever registered those stairs. Then when I go outside and walk up a small slope it registers that just fine.