but am I missing a hidden meaning behind "AR"
I'm sure you know what it really means, but to educate everyone else on the board...
The AR10 was the original "AR." The reason it had "AR" was because it stool for Armalite model 10. Then they created a new rifle with Eugene Stoner that fired the 5.56/.223 round. They designated that rifle as an Armalite model 15, or AR15.
You're welcome for your daily lesson.