Slitherine

So I'll admit I am no expert andI am not trying to be pedantic, but I am playing Morning Sun and purchasing troops and noticed that "Marines" are located in both the IJA and the IJN flags. So I did a little research and they, the marines were apparently part of the Navy, makes sense, in a branch called the SNLF. Anyway I am just curious as to the decision to make them available in both the Army and the Navy. Nooo I am not calling for Slitherine to fix it just wondering about the logic behind it.