Shotgun consumes only 1 ammo yet produces multiple pellets per shot. (btw - grenade launcher consumes multiple rounds of heavy ammo per shot to produce a single grenade)

I wonder if it's a bug or an oversight - for a shotgun.

That's what shotgun shells are like in real life.  They fire buckshot not solid slugs.  Buckshot is basically uniform sized ball bearings packed into a shell with gunpowder behind it.