My DS7 rarely asked "why" questions, yet he knew how everything worked. I explained things a lot, so maybe he didn't need to ask.
This sounds like DS2.25 and me. I don't hear "why" so much as "how do you think this works...", "what is meant to happen when...", "where does this come from...", "how is this similar to/different from X...", "what would happen if I/you did X...", etc.
Another big habit of DS' is to state a hypothesis and go about testing it. Sometimes these are big proclamations like, "I think there is no gravity in space", or little things like, "I think red and blue make pink". We spend a LOT of time discussing what information we'd need to test our hypotheses and, where possible, modeling things out with blocks or drawings. I scaffold with extra information to help DS move from thinking about his hypothesis to testing it, so that may cover a lot of the "why". A few weeks ago we acted out planetary orbit in John Cleese style silly walks. Answering the "why" just happens organically.
And as for the Ruf levels, I think they're a general guide. For example, for ages DS had zero interest in puzzles. Then, a few months ago, he started randomly doing 24+ piece puzzles when bored, and quickly (like under 5 minutes). Now he's totally disinterested in them again. It's so individualized.
