Very few hidden/deleted member posts are actually from hidden members. The hidden/deleted member posts are from people who posted and then deleted their accounts some time afterward. Note the dates it was posted. Most hidden/deleted members posted a long time ago and are not anymore members of this site.
If you post while hidden, it will reveal you again once you unhide your account. Not sure about that, but that's how I understand it.
Also some people probably make fake accounts just to post controversial topics and then delete it so they don't take the heat. But that's just gossip.