I think the confusion is on the fat...well there is ground fat, and there is fat that is marbled in the meat naturally. The meat company does have a way of making the hamburger that gives you the 70/30 ratio. Not the same as the ribeye marbled steak.
http://firstwefeast.com/eat/guide-to-burger-blends-with-tom-mylan/
I spent two years back in the late sixties working in an old fashioned meat market. The lean meat is taken from the animal and weighed. Then the amount of fat wanted in the mix is weighed and added then mixed and ground. The ratio is typically decided on the intended end use of the hamburger. Problem is that most people today have no idea the uses for the different mixes. And don’t let anyone kid you. Hamburger meat is most generally NOT taken from a prime steer or heifer. It’s taken from a cull cow.