See above....
not to mention the VIN's changed to reflect the change from early to 'official' 1965 models.
Ford Marketed the Mustang as a 1965 model year even though it was released about 6 months before the normal model year cars are introduced.
If you want to get into VIN numbers we can go there too. The first number of the vin signifies the model year of the car 5 for 1965, 6 for 1966, 7 for 1967, etc. followed by the assembly plant F for Dearborn, R for San Jose, and T (IIRC) for Metuchen, the next 2 numbers signified the body style either Hardtop, Convertible or Fastback, the next letter signified the engine code. The only engines available in the original 65 models was the U code 170, F code 260, D code 289 and K code Hi-Po 289 (which was the only carry over engine to the later 1965 models) followed then by the vehicles sequence number. The only thing that I have seen change from the VIN numbers of the 1965 models is the engine codes as in 1965 Ford revamped the entire engine lineup only leaving the K code 289 as an engine option.
As to why Ford chose to base the first 5 anniversaries off of the concept of a 1964 1/2 and than change it to the 1965 model I personally do not know that answer. If I had to guess, I would say that for such an important part of Mustang History Ford decided to go with the beginning model year rather than the year that the Mustang was first introduced. Plus this also gives them more time to work on the cars driveline and suspension as well as special edition models that are rumored to be introduced. Looking at how aggressively Ford pursued the Boss 302 program to make sure it was a true successor to the name, I am confident that Ford will make the rumored GT350 and Mach 1 more than just a sticker and badge package.