Because they are set up to register for large slabs of metal, not bodies.
Gatsos can be triggered by bikes.
IIRC it comes down to the prosecution guidlines (the +10% + 2mph you alluded to). Most fixed cameras are configured to not trip unless the 10+2 is exceeded, so you'd need to be exceeding 36mph at the start of the trap to trigger it. [/off topic]