The algorithm for motion detection typically looks for a change in the average pixel values in the coordinate range you've selected. A very low sensitivity (such as 1%) should require a very very large change in values to trigger an event.
Motion detection on my DCS-932L seems to stabilize at around 20%, and that's a in dynamic environment with shifting light patterns from a window casting light into the area being monitored.
I'm not sure if this will affect anything, but under Setup -> Video, did you try increasing the Resolution, FPS, and JPG Quality. If the motion detection algorithm is executed after the image quality/FPS pre-processing, these settings may impact the results. Also, is the camera in night or day mode when these detections are occurring? Try testing the motion detection in "Always Day Mode" and "Always Night Mode" and see if the results vary.