My sense use to accurately count stairs. But now it is counting or adding a ridiculous amount of stairs. It doesn't matter to the weather either no matter what the weather. It's still adds a ridiculous amount of stairs so I know it's not the air pressure.
See this thread...its still very active:
https://community.fitbit.com/t5/Sense/Sense-is-counting-too-many-floors/m-p/4579274#M6841