From what I’m reading in this post Comment on OP comments it would appear that the angle for the spread of shotgun pellets reduces with perks that give accuracy bonuses.
This seems nonsensical to me, as regardless of how accurate you might be when firing a shotgun, it’s spread of pellets is dictated by the gun itself (specifically barrel type and length), not by the ability of the wielder.
Therefore, when firing a shotgun, the accuracy of the centre point for shotgun pellets being fired could improve, but the angle of spread from that centre point should remain constant.
In general, firing a shotgun should set a centre point, and then spread from that point, I think the problem comes from each pellet being treat as an individual projectile and its trajectory being calculated as such rather than treat those pellets as a collective shot.