GPS issues may occur during an activity. This can be caused by the phone's operating system attempting to preserve memory and battery, as well as accuracy of the GPS signal.
If you have a weak signal, then discrepancies can occur. In this case the activity may report a different distance than you travelled, and the STEPtember app cannot ‘fill in’ the missing data.
It is also common that different devices can show slightly different data for the same activity. Issues typically fall into three categories:
GPS drift: The GPS track generally follows the shape of the road/path but with much less precision.
Lost GPS signal: If the GPS signal is lost and later re-acquired, the pre- and post-signal-loss points will be connected with a straight line.
GPS bounce: A 'jumpy' GPS signal can cause your activity to report more distance than you travelled.