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That's assuming he's drawing this live. Who's to say he hasn't been planning this from Day One and has been drawing the frames all along.

I know, it's a stretch but ... what if?

Randall has shown the capability to draw enormous amounts before. Don't forget http://xkcd.com/1110/ . See http://www.wired.com/wiredscience/2012/09/how-big-is-the-xkc... for an analysis of how big 1110 is.