Well, John Giant is kind of strange. On one hand, his clothes look like one of those real life admirals'. On the other hand, he likes to shout, which isn't something a sophisticated person would do every time he/she appears on-panel. But, he has like a dozen medals and I remember no one else who has any (Onigumo and Momonga has one only). He also wears an awesome hat with a giant feather.
I've always seen John Giant in a military way, and as such, the shouting fits.
Think of a military man that shouts and you don't get a "sophisticated person" but do get the image of a drill sargent.
Hey, what if sometime ago, the WG tried to turn giants into obedient soldiers and they only succeeded on certain islands inhabited by giants? Or just one? And killed lots of others in the process? And that's why now the marines have only 9 giants?
That's a lot of question marks…
That is quite possible. It would fit into the World Govt.'s desire to really rule the world (let's face it, they don't )