It likely has to do with the way they're sensing the teeth on the trigger wheel.
Hall effect sensors (most aftermarket and later production ignition systems) behave more like points where it detects the presence or absence of a tooth on the wheel. Basically, it's a metal detector that indicates when metal is close to the sensor (i.e. when the tooth is directly in front of it) or when it's further away. The output of this type of sensor is a nice square wave.
Variable reluctance sensors (used in HEI distributors i.e. pickup coils) detect the *change* going from a non-tooth/valley area to a tooth and tooth to non-tooth. Also, the amplitude of the signal changes as the toothed wheel spins faster. If nothing is spinning, there's no signal from the VR sensor. It doesn't know whether it's sitting on a tooth or in a valley between teeth. The output of this type of sensor is something of a sine wave (depends on how close the teeth are) that oscillates positive (when a tooth transition is detected) and negative (when the valley transition is detected).
With a hall effect sensor, if the key's on while the engine is off and the sensor detects a "dwell" state, it will behave just like points and start charging the coil until something gives. With the VR, nothing is going to happen until the engine is moving.
Later model ignition systems have moved to hall effect engine position sensors. But, they also have computers that can look at the history of the signal to determine whether the engine is moving or not.
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