Typical attempt on Moore's part to get rid of improbable coincidences (like naming the Marvel U Earth 616, instead of Earth 1 or 2 like in DC).
Actually, I don't think that was an improbable coincidence. I think Earth 1 was called that simply because it was the earth that Barry Allen resided on (and, obviously was therefore first aware of) and, when he discovered another earth, he called it "Earth 2."
