There could be a more simple explanation:
As more and more accessories began being introduced to vehicles, the ignition/start switch sytem had an extra terminal added, +15A, A as in the letter, not Amps. This terminal was added to reduce current comsumption whilst the engine is cranked and circuits which are not essential to actual engine start, such as supply to the heater, cigarette lighter/power outlets etc are connected to it.
Terminal +15, which supplies the ECM and other necessary circuits, stays connected when in the crank position, whilst +15A disconnects....