I'm honestly not smart enough to explain the distinction but I'll try. We're one of the two terrestrial apes now, but in the Miocene we've got a whole grip of weird apes running around Africa and Eurasia on the ground, and they're all more closely related to gorillas and have similar adaptations.

We share a common ancestor with chimpanzees more recently than with gorillas. Either the mutation to a more gorilla-like clavicle hadn't happened or was not enough of an advantage to be selected for as we evolved away from proto-chimps.