Connecting your phone as a device is supposed to only use phone's step's count when you are not wearing your fitbit tracker. When you are wearing your fitbit, it should have no effect. However there have been a few cases where people have noticed a huge sudden jump in step count (many tens of thousands of steps or more, added all at once), and the common thread seemed to be that they had their phone connected as a device, leading to an unverified suspicion that it was caused by a bug in that feature. Actually, if a phone were always kept in your hip pocket, it might get better total step count than a wearable on wrist, but fitbit on wrist is getting so much more info, including heart rate. No tracker worn on wrist can measure steps precisely because it doesn't really know what your legs are doing. It can only tell what your wrist and arm are doing and infer steps from swinging of the arms that look like movements that go with walking. If you play the drums or do a lot of knitting or such, you will probably get a lot of extra false steps and be better taking it off during those activities. But otherwise, most people get a few extra steps sometimes and get a few steps missed, but over the course of a day, they pretty much even out. And remember that heart rate is really a more important factor than steps for most measurements, including calories burned.
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