the decission to go around is made at the Decission Altitude. this at some airports can be as low as 250- 300 feet.
jets have a lot of inertia in them so when the pilots decide to go around it will take time for the aircraft to catch up with them. jet engines also take time to spool up when a power comand is given to them so this does not help a go around.
both of these together may have the aircraft drop down to the runway before the aircraft actualy starts going around, even though the decission would have been made a few seconds ago.