OK this probably isn't really relevant to the 777 but if one is doing a PFL in a light aircraft and deploy full flap early (ie before you know you can 'make it') the glide angle becomes steeper and the plane sinks faster for a given speed, because in general the last stage of flap adds a lot drag and not much lift (if any) or decrease in stall speed. If one subsquently raises the last stage of flap (apon seening the approach angle shallow) the aircraft starts to glide better again. I don't know but I'd have thought that raising the flaps a bit would have increased the gliding distance in the case of the 777.