Stupidly, yes. Guessing they're assuming the gloss value will stay relatively well compressed in the alpha, with AO getting next best compression in the green. Metalness doesn't need that much to it, so it gets next best in the red. I've had to hack the Standard shaders to use RGB rather than RGA and that's a real pain now…
If you are using Unity 5.4+ there is a new slider that is a smoothness override slider now. It's meant to tweak the smoothness value in the alpha of the texture. It's a multiplier, so it multiplies it's range(from 0.0 to 1.0) against your alpha channel to reduce overall smoothness. Something important to note between…