The build date for my 1971 Dodge is listed on the fender tag as Dec 11 1970, but the VIN decal on the door reads 1-71 as the build date. Is it normal for these dates to be different or does it depend on the assembly plant or which part of the year a car was built?

The date on the fender tag / broadcast sheet is the Scheduled Production Date - not the actual build date unless it is just a coincidence.

The SPD is the date they hoped to build the car when the order first hit the system.

Your car was built later than the SPD for some reason.

See this thread for more info;

https://board.moparts.org/ubbthreads/show...e=0&fpart=1

Is that uncommon or something that simply happens from time to time?

VERY common.

Some cars were late & will have the door sticker dated the next month.

Some were built early & will show the prior month.

71R/T is this a Challenger you are talking about.

It's a Charger R/T.