Any GPS device will show your position moving around and an associated speed due to the inherent limitations/precision of GPS. These are usually on the order of 5-15 meters and a few miles per hour when stationary. A low battery will not cause or affect this as the unit will work correctly until the battery reaches a limit and then shut off before operation could become erratic. Garmin provides for a wheel sensor to help overcome this GPS limitation. It preferentially uses the wheel sensor for speed and distance which makes distance over tortuous paths more accurate, short-term speed more accurate, and the auto-pause function more reliable.

Don't worry, be happy. Set it up and use it on your bike. It will almost certainly work properly and well for you. If not, Garmin has very good customer service and honors their warranty.