As we learn in Swan Song, Columbo served in Korea, not in the Military Police, but, as he says, "KP (kitchen patrol) mostly." Since he did a lot of time doing KP, we can assume that Columbo wasn't very good in the military and never bothered to learn military time.
Yes, it could be the fact he was mostly in KP or it could be another example of the shrewd lieutenant deliberately playing dumb which is one of his most notable character traits.