it IS vented properly it drains, a blocked vent or no vent would cause the drain pupe to "gulp" just like a 2 litre pop bottle gulps if you pour it out fast ( invert the bottle and see the bubbles ), the vent in this example would be the equivilent now ( using the pop bottle example ) of poking a hole into to allow air into the bottle, a blocked vent will also cause the drain to smell really badly as it will syphon the p-trap ( the vent in THIS example is the equivilent to poking a hole into the syphon and breaking the syphon as air Can freely pass and is not drawn by the displacement of water )
So, its likely not a vent . I would surely not worry about this.
What mr david referrs to is called the "Corriolis effect" in essence, when more mass ( water ) is drawn into this effect that produces a low pressure zone ( the center of the swirl ) and is allowd to gain momentum it will draw faster and create a greater low pressure area.
Hope this helps