It wouldn't be that unusual, since as you know, the door date is simply a 'guess', based on supply chain and priority.
Look over the rest of the car more carefully, anything with a date stamp, to help paint a better picture of when the car was actually made. Glass, sheet metal, under-dash components, etc...
What is the casting date on the block? Also, if you weren't aware, there is another stamped date, on the oil pan mounting surface that usually is the same as the date on the machined boss on the top. Head date castings, intake manifold, balancer, distributor, carb, all would be dated also. If all of those are before or close to the 6B date, can help clear things up.