I thought the detailed records were kept, but all of the muscle car era records were lost in a fire in the late 70s, early 80s. So that all that remains are the compiled figures as show in the above link. I also know Cerebus gave away and destroyed a lot of records as well.
I also thought Galen's database was of cars that he had hijacked vin info on or had vin info submitted to him by owners, ergo, his 1 of 20 claims are based on cars in his database, not total production figures.