I do love that deeper speaking voice!
But in my opinion, I think the main reason he often talked in a softer, slightly higher voice was at least in part due to shyness rather than "faking" anything. I'm not 100% sure on that of course, and I can't confirm it either way, but the truth is that Michael was a rather shy individual, he even took anxiety meds which is saying a lot. And speaking from my own struggles with it, I have a tendency to do the same thing, which is why I have this opinion. Then again, honestly even if he was "faking" anything, I wouldn't really care either way.
At the same time, people treat his deeper speaking voice as if it's some sort of undiscovered, shocking secret, which utterly baffles me LOL because it's out there, as we can see. And I've known about it for quite some time already. I do agree that many people tend to over-exaggerate Michael's higher voice in imitations of him though, then again they do that with everything he does so it's also not entirely surprising.