The curved versus flat roofs may have something to do with it.
I have the Z-Stuff dual semaphore bridge and can confirm Mike's observation about color having an effect on triggering the sensor. An IR detector uses reflected light (as in Stan's diagram), so the color of an object may change the intensity of the reflected light back to the sensor and cause the detector not to trigger. Black absorbs all light so the reflected light back from a black freight car may not be strong enough to trigger the detector.
Note in the attached video how the right side track semaphore triggers almost immediately after the silver roof of the Warbonnet engine travels under it, while the left side track semaphore does not trigger until the light grey B&M Hopper passes under it.