OK, that was interesting - I attempted to use my Gmail account as above, but even while going to my Gmail webpage in another browser window and waiting for the Discourse generated verification email to turn up there, I was notified via my normal administrator email address about a new sign up to review.(and queue was empty as before). I had mistyped the address so never received the email to my Gmail account, so never verified.
So unless this is a coincidence, perhaps administrator notifications are jumping the gun? Though that would not explain why EVERY notification involves exactly one missing person in the queue - presumably there will not always be a failure to verify email address by one applicant but not the rest.
** Edit
Subsequently, I corrected my Gmail address in that incognito signup window and resent the verification email - which then turned up at my Gmail browser window. I verified this using the verification url in another incognito window successfully. No subsequent notification was sent to my administrator email address, so I can only assume the first notification was indeed associated with this new sign up attempt.
Using yet another browser window, I visited the site again using safe-mode, signed in as administrator and saw the same queue still with one person to review - but this time my new test account was visible and could be approved.
Does any of this shed light on the issue?