This issue occurs when using a Motion Encoder Cart Receiver (MEC-BTD), with a LabPro Interface. The zero point is not maintained between experimental runs.
This is caused by an interaction between the encoder system and LabPro.
This behavior cannot be changed when using LabPro. As a result, if you need the origin for the position data, re-zero before each run.
When used with a LabQuest interface (any model), the zero is maintained between runs.