THAT VID OF THE SPEEDO COULD HAVE BEEN DONE ON THE SPOT

when i had wennys escort van at brunters for the fast ford thrash the speedo was right round pointing at the bottom & i was only recorded at 130 something so how can they use a vid off the net as proof of speeding

i chuckle to my self every time i get on the motorway & my speedo says nearly 80 but the gps speed is 70 speedos are always wrong making people think they are going faster than they are