I am new to Joomla and Kunena, but i found that in my "Global Configuration>Permission Settings" for "Registered" both "Access Administration Interface" & "Admin Login" were set to "Allow"
For my particular website setup i am the only Admin to alter/change website, so i turned both to "Denied"
Noticed whenever i switched "Access Administration Interface" back to "Allowed" the username in Kunena would change back to "Site Administrator" with all the admin forum privileges enabled.So now i leave it at "Disabled" and this fixed my problem.
Not sure if this helps anyone but it put a halt to my "All Users Show As Admin Logged In" issue.
cheers,