The numbers used to replace the letters ‘somewhat’ resemble those letters. I guess the mind automatically fills in the correct letters based on the shape?
An episode of Top Gear touched on that (reading based on shape). They had on the designer of British road signs who changed lettering from all-caps to upper and lowercase, against convention. The reason was that the brain can more quickly match the overall shape of the word after is was seen so the roadsigns could be read more quickly.