The other possibility (for example in the case of the swimming qualifiers) could be that ticket holders decided that it was more interesting just to hang out in the Olympic Park.

FORBES: The Olympic Ticket debacle

For example, just before the start of spring training, he tweeted a photo of himself shirtless in a swimming pool, buoyed by a tube and a pair of floaties, with goggles over his eyes and a snorkel lodged in his mouth.

WSJ: The King of the Accidental Phone Call