I've only just got my watch so only have a few days data but I'm noticing that it's not correct. For instance yesterday I did 13.5k steps which seems right but then it's saying only 31mins of activity. Another day it was 1.5k steps (day I got it) and only 3mins activity!
My sleep recordings stopped about 2.30 this morning but I can't see any rhyme or reason as to why - I didn't get up and walk anywhere and the watch was still in place. I did have disturbed sleep but it was showing as no disruptions at all until it stopped at 2:30.
I'm using the official Withings App but I'm wondering if there's another one about that's better. At the moment I'm pretty disappointed with the App and watch as I mainly got it for steps and sleep tracking.