Easy test,
Take a seat of the floor, or on your knees.
Now drop an elbow from 12, which is straight up like on a clock, and drop it down, to where the 6 would be on a clock.
See how easy that is?
Now lay on your back
And do the same muscle motion.
Not the fucking same.
Also people fair to realize that a 12-6 elbow is damn near indefensible. It splits guards and it has a way of finding itself into eyes. It usually can only be used when you are ontop of someone in guard, or defending a takedown.
If you are in someones guard and they are hitting you with 9-3 elbows, well, you have the advantageous position, you could easily stand up and not get hit with those elbows.
See what they are legal?