It’s a matter of physics — if you pour with the lid on the bottom and the carton is full, milk comes out from all sides, so it comes out with increased speed. As the liquid flows out, the carton tries to equalize pressure, so it lets air from the outside enter through the opening. The incoming air could disrupt the flow and cause the milk to “spurt.”

Pouring with the lid on top means there’s no pressure or milk from above, and there’s enough space for the air to enter. Doing this will give you a smoother, more continuous milk flow.