When I cut open my ceiling to fix a plumbing issue I didn't know what I was going to find exactly, but I had a pretty good idea of the possibilities. My estimate for how long the fix would take was pretty close.

Software is the same thing. There are unknowns but there aren't unlimited possibilities.

It's a great analogy, but I think it's the kind of analogy that works on the surface and not in the details most of the time due to a) much more codification of practice in construction and b) more "fixed" knowledge of what's likely based on location and build era of the home, whereas software is much more dynamic and often subject to individual whims of developers or management.