Great work @paulag!
I believe that a good percentage of these accounts were created automatically by some software, but it'd be very hard to detect those accounts.
I also believe that many people register accounts and forget all about them; or at the very least lose interest in Steemit; as some people had to wait for days if not weeks to get approved; I think that's a major problem that puts off a lot of new members; but we should wait and see how this issue will be addressed in the next HF.
A starting point in detecting these accounts would be voting, and delegated steem but no posts.