Restore Database using GUI - Wrong file to restore
Chances are something in the backup history tables got out of sync and the UI is going to the last "consistent" full backup.  If you're really interested in why it's doing what it's doing, start a profiler trace limited to your account, walk through the restore steps in the GUI, and review the commands captured in the trace that show what the UI is doing behind the scenes.  This will get you the definitive answer you're looking for.

If you'd rather just move past this, you can clear your backup history via EXEC [msdb].[dbo].[sp_delete_backuphistory] @oldest_date=getdate() (depending on the last time you ran this, you may want to clear it out a month at a time) and then take a new full, etc.  I would suspect this will reset the GUI to use the proper backups going forward.

Finally, another option is to run this script authored by Wayne Sheffield.  It may provide some more information regarding any issues with the backup chain.  I didn't come across this until after I posted this answer originally, but hopefully it helps someone else out in the future.