Why is the supply air duct not directly behind my vent?

I was investigating why is one of the air vents so loud when AC is running. Turned out the air duct is not actually behind the vent, but to the side of it.

This is looking from the vent towards the duct: https://imgur.com/LoVmJLB

Video for better context: https://imgur.com/a/xije77u

Is this a common thing? The casing for the grille (left of the picture, the white thing) is restricting air flow a bit and making the whole vent kinda loud.

I am thinking to get a dremel and just cut off the extra casing that's serving no purpose

Edit: mixed up return vs supply. This is a supply vent.