1. Build dates are approximates. Fisher had no reason to be rigorous about them.
2. The answer is right there in the monthly VIN listing that you referenced (http://www.camaros.org/geninfo.shtml#HowMany) :
Due to several limitations the VINs in this list will not necessarily correlate exactly with either a specific calendar day or the build week on the cowl tag. The list data for some months deviate significantly from actual build dates, while other months correlate well.
That needs to be bolded on the site too.....