[Neus]

It gave you that amount because that's how damage it does without the 'bonus' damage from the gun attack.

It's not really a bug, in my eyes, because;
A) Guns get attack scaling on initiated attacks only, not counter attacks.
B) Guns weren't ever intended to be used with Riposte.

At this point, it's an old issue that gets reported a lot and I'm tired of seeing it, so I'm tempted to just change Riposte to only work with melee weapons, but then I know someone is going to report it as bugged when the mutated gun they're using either doesn't trigger Riposte (because it's being treated as a gun), or does trigger but doesn't get its damage scaling anyway.