A lot of the good, topical questions on SuperUser fall in or around the same areas of expertise, so if someone is making a lot of answers (and the answers are good/valid/upvoted/etc), chances are they simply don't have anything they need to ask that would be answerable and good quality and topical.
On the other hand, these users may have accounts elsewhere on the SE network where they primarily ask questions and post few answers. I'm that way on sites like StackOverflow and Mechanics and Pets, since I'm not as much of an expert in those fields, but the types of questions that I would ask on SuperUser, I'm already able to answer myself -- either by googling, doing a little research in technical manuals, or just know it off the top of my head.
On sites like StackOverflow the Q/A ratio could be more balanced for high-rep users, because the topic range of SO is much greater than anywhere else on the network; you may be an expert in C++ programming for GNU/Linux, but if you suddenly start on a project involving Ruby on Rails running on Windows, you might be chock full of "noob" questions that are topical for SO.
That kind of thing is less likely to happen to high-rep SU users because the site is more focused, unless, say, you already know tons and tons about Windows, but you're new to Linux, so you could ask your Linux-related questions here. But a lot of our advanced userbase tends to use both Windows and Linux quite a lot, so even that situation is somewhat of a niche.