It's a habit to get the whole picture when someone asks for help with a softener. It's easier to help when everything is in context cause it's hard to see the situation through a monitor.
As soon as someone mentions, even in the vaguest terms, that the water doesn't stay soft asking for the water conditions, # of people, and the SFR is a reflex because if the softener isn't sized correctly there almost always is hard water, even intermittently, before regeneration.
Your difficulty is that you want to know the "why" along with the "how" and unless you run into an educator with expertise in the specific field you're asking about you're not going to like the "why" answers. Few of us are teachers but we know the "how" and that's what we give you. Want your softener to work, then be quiet, listen, and say thank you.
I don't need to know what Purolite says, I've contacted one of their reps and get my information from him.
My water conditions are similar to yours with the same two people in the house and I use one bag of KCl a month but what do I know?