The most common cause of this problem is not running stsadm -o preparetomove prior to moving or upgrading the database. 

In case if you don't know there is a hidden list named "USER INFORMATION LIST" which displays all the users of the site (collection) {MOSS / Sharepoint 2007}. And there is a page "Simple.aspx"  which show this list, the url for the page would be http://URSITENAME/_catalogs/users/simple.aspx normally no one has access to this list (except for the admin) and as the list is hidden it is not viewable in the "view all site contents" too. You will have to open you site in SPD and then click on _catalogs + and then you should see the "USER INFORMATION LIST" under it. 

WSS sites (including those within the sites collection of a portal) have something called a User Information list that keeps a limited amount of info about it’s users (login name, email, display name).  When a user is granted access to a site, their info is put in a User Information list.  When a user creates/edits an item in a list SharePoint creates a link to the user’s record in the User Information list so when the user is shown in “Created By” and “Modified By” indicators, the Display Name from the User Information list is shown

 

SharePoint Portal Server has something called a Profile Database.  When you first visit a portal, SPS checks to see if it has a profile for you and if not, it creates one on the fly with your info from AD.  You can also configure SPS to automatically populate the Profile Database with a full import, an incremental update, and optionally set it to pull objects with a custom LDAP query. 

 

Here is the summary.  When a user accesses a site collection the first time, they are added to the profile.  Howevere, a property named tp_IsActive  in the USER INFORMATION LIST is set to false until the user interacts with the site by adding a document or clicking a link.  When this property is false the user information will not be synced by the jobs WSSProfileSynch and WSSSweepSynch .  Thus the inconsistency begins to develop.

 

A lot of use information can be obtained from http://msdn2.microsoft.com/en-us/magazine/cc163287.aspx or http://www.21apps.com/2008/02/user-profiles-why-do-my-changes-not.html.