People:
G.I. Jane had Captain bars on her collar but they called her lieutenant, why?

She is a Navy Lieutenant which is equal to Army, Marine and Air Force Captain rank (0-3). Navy Captains are equal to Army, Marine and Air Force Colonels (0-6). SEAL school is a Navy run operation, but some other branches get to send service members to it on occasion, IIRC, there is an Army sergeant in her SEAL class.

Here is the Navy to all other rank equivalences:
Ensign=Second Lieutenant
Lieutenant (Junior Grade or J.G.)=First Lieutenant
Lieutenant=Captain
Lieutenant Commander=Major
Commander=Lieutenant Colonel
Capatin=Colonel
Rear Admiral (lower half)=Brigadier General (one star)
Rear Admiral (upper half)=Major General (two star)
Vice Admiral=Lieutenant General (three star)
Admiral=General (four star)

In any case if I remember correctly that was one barf-up of a movie. Demi should stick to stuff like Strip tease. She was pretty bad in that A Few Good Men (of course without Nicholson that movie wouod have been major league stinko)