To add to mjm's post with an example; my father in law lives in a small(ish) block of flats in Minami Azabu. The residences are split into two blocks which share the same stairs and elevators and are connected at each floor.
I don't recall the whole story, but the gist of it was that one block was built to higher specifications than the other block. Knowing this, one starts to see a few differences (certainly a few more cracks in the inferior build). But I can't imagine many people guessing the discrepancy in quality just by looking, the two blocks appear to be the same.