There is indeed some inconsistencies here. I've looked at a few store models and on some they are solid, and others they're loose. This is spread across both 13" and 15" models. Interestingly, they're always consistent at the same store. Store A would have solid buttons for both models, but store B would have loose buttons for both models. Not sure if it's a batch issue or just pure coincident.
Regardless, I'm not too fussy about it, although the looseness does make it feel a bit cheap.