I just read a little about Custer and apparently he was a Brigadier General of Volunteers at 23. This rank did not aparently apply to the regular army.
"At the end of the Civil War (April 15, 1865), Custer was promoted to major general of volunteers, but was reduced to his permanent grade of captain in the regular army."
http://en.wikipedia.org/wiki/George_Armstrong_Custer
Never knew there was any sort of differentiation