In the past I have the same problem, the AppStore offers a OS X Update but this was already installed.
Finally the problem comes from "Spotlight" because I have a "Time Machine" Backup Disk connected and this disk was not successfully indexed. In my special case, the indexing was never successfull. This results in the problem that the AppStore finds a old availabe version of the OS X release on this drive.
Here the way how I fix the problem (using Command Line mdutil tool):
The Problem:
# mdutil -s -a
...
/Volumes/TM Backup/Backups.backupdb:
Indexing enabled.
Scan base time: 1970-01-01 01:00:01 +0100 (1376397204 seconds ago), reasoning: 'Not set'
...
# mdutil -v -E "/Volumes/TM Backup/Backups.backupdb"
/Volumes/TM Backup/Backups.backupdb:
Indexing enabled.
Scan base time: 1970-01-01 01:00:01 +0100 (1376397283 seconds ago), reasoning: 'NewIndex'
NOTE: wait some minutes after indexing has been finished
# mdutil -s -a
...
/Volumes/TM Backup/Backups.backupdb:
Indexing enabled.
...
After that the problem was gone. If I open the AppStore the already installed update is no longer visible.
Regards,
Daniel