Hmm, that's weird. I can't spot that part in the wiki about Carth on Wookiepedia. Instead I found this:
In 3,964 BBY, he served aboard the Inexpugnable-class tactical command ship Courageous as a helmsman and was promoted to Lieutenant.
[...]
Onasi eventually found himself on board the Endar Spire as mostly an advisor, carrying Bastila Shan, a Jedi whose abilities were vital to the success of the war effort.
And that's where the story of KoTOR sets in.
For his part in the defeat of the Sith forces, Onasi (along with Revan and the rest of his companions) was awarded the Cross of Glory.
[...]
Some time after playing a part in the Battle of Rakata Prime, Onasi was made an Admiral in the Republic Navy.
And that's it. Nothing more said about his rank.
Source: Link (
http://starwars.wikia.com/wiki/Carth_Onasi)