Do we know the gates from which the flights in question were departing from? 74-76-77-75 (in clockwise order around the end of the terminal) are all linked by a sterile wraparound corridor to FIS after the BP scan at each respective gate "door." It's quite possible that HND was boarding at 77 while IAH was at 75B next door, the pax in question scanned his BP at 75B, but somehow took a wrong turn after the BP scan and walked down the jetway for 77 instead. There should be safeguards in place to prevent this, but they can fail from time to time.

ETA: I see the article states the IAH flight departed from 75A, which is down the hall from 75B. The scenario I posited above is still in play.