Build date in door decal is meant to identify the build date when looking at parts in microfiche. Its printed with your VIN and identifies the way BMWAG looks at your build month/year.
In your case my guess would be that your car was built with a least one part that was before they started August 2012 production.
I've done plenty of searching on
www.realoem.com and have not yet come across parts that changed with 8/2012 production. 7/2012, YES, but not 8/2012. There must be something, somewhere in that parts catalog that changed in 8/2012 that is not in your car.
My guess.