On the other hand...
Data: Saved Earth from assimilation by the Borg. One year layer, may have saved the Federation by ending Romulan involvement in the Klingon civil war. Still a LCDR.
Deanna Troi: Took 23 tries to kill a facsimile of Geordi LaForge. Promoted to CDR, and Data has to call her "sir".
If Data was over-promoted, so was Troi.
Come to think of it, Troi was promoted the very moment she passed that test. Could it be that passing that test is a necessary condition for promotion to commander rank or higher? (That is, all those that were promoted to commander had passed this test at some point in the past, but Data either never had taken, or never had passed that test?)