I think I know how he did it...
I think I can give you an answer on this. I almost always rate the puzzles and twice I have been unable to rate a puzzle that I just finished because I rated it before. I'm guessing that's because I played it before! I know from reading the posts that some people make separate spreadsheets when playing. My guess is that they keep their old answers and compared the columns on the first page to previously completed puzzles and all they had to do was fill in the green marks to be done!
I know some people might think this is cheating, but until all the bugs are worked out of the site, I call it pretty smart!