Yep, this kinda reminds me of the great Superbird production debates back in the 1980s. Everyone though there were about 1,920 cars built, but then cars with VINs that were never recorded by Chrysler started popping up everywhere. Now the number has grown to 1,935 or 1,971 depending on who you ask and there's still cars that show up from time to time that were made off the books, usually for race teams and the cars somehow made it to the street.
Govier's database is considered to be the best, but even he admits it is nowhere near complete. This is especially true of police cars since they were usually made by contract and tracked separately. For example, if you search for a 68 Belvedere with a 383-2 or 383-4, RK41G8 or RK41H8, which were what the LAPD ordered en masse in 1967 and 1968, that same database shows no results found. This is why collecting police cars is such a major PITA.