Well I could explain. Do you know anything about wormholes?
Seriously, ground speed is the speed of the plane relative to the ground. Airspeed is the speed of the aircraft relative to the air around it. So if the air is moving in the same direction as the plane (wind, jetstream, etc...) relative to the ground the ground speed can be quite high despite airspeed remaining well within acceptable limits. This phenomenon is of course referred to as "tailwind".
691mph=1112km/h. This is in the high range but not unusual. Jetstreams can move at several hundred mph.
[Edited 2004-12-27 03:56:44]
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo