It is all about how the designer approached what they wanted to do. The magnetic pickup is versatile and reliable and only a 2 wire set up. The box can condition the input signal, and in the end whether they use an PNP or NPN transistor determines whether it triggers off a rising voltage or a falling voltage. Those 50 year old systems still work fine from the distributor portion, even when left to the weather. Clean right up and work. Chrysler was first, other had the advantage of seeing what they did, then follow-up along with improvements that where occurring in the electronic field.