Conversely, I thought it was because the timelines are a little different. Black Lagoon is the earlier series (2002 it looks like...? I don't know if that's the first bit of it) and is I believe set in the 90's? Where Jormagund came out half a decade later and was I think more concerned with being bleeding-edge stuff because of the weapon-dealer angle.
I'm not enough of a weapons geek to be able to tell you for sure though.