Hey,
the pollutionreduction is calculated multiplying emmisions (as you listed) by their respective energy usage and crafting speed. So the values are as follows:
MK1: 12/tick (720/s)
MK2: 84/tick (5.040/s)
MK3: 1500/tick (90.000/s)
It is intended for the machines to keep working, even when pollution is low. For this very purpose, I've made another mod. It's called "Pollution detector" which adds an entity which you can use with the circuit network to control your filtering machines: https://mods.factorio.com/mod/pollution-detector
Greetings,
Schorty