If you click the red "Learn more" link on bottom of the "% Ontime" page you get the small print:

"% Ontime Arrival: The percentage of time that the flight arrived "ontime" for the most recent month reported to the Department of Transportation (DOT). DOT defines a flight as "ontime" if it arrived within 15 minutes of the scheduled time shown published in our Computer Reservations System."

Also, isn't it Flight 888?