I would guess that the first day of production wasn't set in stone. Tooling could be held up for a few weeks or a car maybe selling really well so they continue production a little longer. Normally the beginning of August is when production starts.(I think) It looks like 72 production started earlier then that.
The SPD(scheduled production date) on the fender tag is just a date the factory HOPED to build the car. It doesn't mean that the car was built on that date. The VIN being 100096 also doesn't mean the car was the 96th one built.....It was just the 96th car to be assigned a VIN number. Cars weren't built in VIN sequence.
Sometime during the 72 model year, the factory changed the VIN decal on the drivers door. Your car probably has the earlier style sticker. The factory added an MDH code to the VIN sticker. MDH stand for month/day/hour. Since it is believed that the stickers were printed close to when the car was at the end of the line, the MDH code is most likely the day the car was completed.
I'll use my 73 as an example....
SPD 927 (September 27th 1972)
MDH 101218 (October 12th...18th hour)
My car was built 2 weeks after the SPD. Some cars would have been built before their SPD's. A lot of the original parts on my car have date codes right around the SPD. This would make sence that the parts would have sat around for a week or two before finding their way onto a car. You can look for original date codes on the parts to get a better idea of when the car was accually built.