Where do airplane variant numbers come from?
Chinese superstition. The same reason Airbus went with the 380 number when 350 would have been next in line.

The Chinese sign for 8 looks like an up arrow (八) and 8 is the lucky number in Chinese. Similarly, 4 (四) is considered unlucky because the word for it (sì) sounds similar to the Mandarin word for death (sĭ).

Boeing and Airbus management court their prospective Chinese customers wherever they can because they expect their biggest sales in the next decade from that part of the world.

Originally, Boeing used three-digit numbers for their subtypes, and the last two digits represented the airline. Pan Am's number was 21, for example, so a Boeing 707-100 made to Pan Am specs was a 707-121.