Agree, a bit jarring, and I don't think it ever quite answers the question either. I saw the title and decided it was a "how many gas stations are there in Chicago?" type lateral thinking question. I must admit without a lot of thought I blurted out an internalised "maybe 1 for every 10 people" (trees not gas stations) and so "500,000". I am wrong by several orders of magnitude. Whoops.