Deliberate mistake: The license plate on Teddy's car is constantly changing. When Leonard reads his card of the plate number, it could end either with 7IU or 71U since his I's and his 1's are written in the same way. When Natalie gives him the information, the car's registration ends in 7IU. Even the car's plate is both 7IU and 71U. Leonard's tattoo of the number ends in 71U, but he says 7IU.
Picture added by Sereenie