Why are there 2 firmwares on one camera (Nikon)?

I updated the firmware on my D7000 today and I was just wondering what the purpose of the A and B firmware was?

Added (1). There are both A and B on the same camera

I'm guessing those were different depending on when your camera was released. The first off the line were probably shipped with the A firmware, while later ones were probably shipped with the B version.

There shouldn't be much difference between these I would think. But if you can select which one to use then since you updated I'd choose "B" and see what happens. Does it change the way the camera operates? What exactly are those changes?

If you need more guidance I'd contact Nikon for further details.