Your post is great, but a picture is worth 1000 words. Do you have a diagram for us, to assist in dealing with these (very necessary) very important technicalities?
Left ribeye on the bone, right sirloin on the bone.
You can see the ribeye is made up of 2 muscles and the loin only one. The large muscle in the middle of the ribeye (the eye) is the same muscle that makes up the entire loin (longissimus dorsi). It is more tender and marbled in the rib area, which makes it tastier.

