Triggering confusion

I have a Simplicam in an outdoor housing pointing along my driveway, on which are parked two cars side by side with about a three feet gap.  The camera does not cover the footway/highway.

1) The camera IS triggered by reflections of passing vehicles in the side windows of the cars.

2) The camera IS triggered by the appearance of the legs of an incoming person at the footway end of the driveway.

3) The camera IS triggered by a person exiting a car parked in the driveway.

4) The camera IS triggered by a person walking along the driveway from the house, immediately they appear in shot.

5) BUT, the camera IS NOT triggered by either car entering the driveway.

So, why is the camera NOT triggered by the motion of a large object (a car), but IS triggered by the motion of a small object (a pair of legs)?

I have tried "Medium" and "High" sensitivity settings with no change in behaviour.  I have tried walking slowly into the driveway - still detected.  I have tried driving in quickly - not detected.  Surely a gross motion should trigger the camera?

Thoughts, comments, ideas?  TIA.