The answer is that some countries allocate mode S codes in strict registration order so that every possible registration has its place in the Hex sequence even if it isn't currently allocated to a registered aeroplane. Consequently the GAS can guess the registration from this strict Hex sequence even if it isn't confirmed as being a newly registered plane.
For certain of the aircraft that GAS is guessing, airliners.net will already have photos from delivery flights etc. If you delete the words 'tie-up extracted' the Airliners net search criteria means that the aircraft type, construction number etc are dragged from the information on the airliners.net photo, even when the info isn't on GAS yet.