That makes an awful lot of sense, in which case the bug is just not checking that the login is 'cached' before displaying the dialog. In a similar vein to another comment, though, why doesn't that dialog detect the state of the 'login cache' consistently across all occurrences?
This only appears to be when logged in as a local admin. Tested with a non-admin account and I cannot unlock the prefpane with incorrect credentials.
So basically they are checking to see if you have credentials already. I guess this is a caching issue since you locked it.