In order to best aerate wine, you want a lot of the wine to be exposed to air. Paradoxically, as the wine gets more air, you'll want to expose less of it to further aeration, and once you start drinking the wine, you'll want to reduce that surface area to volume ratio even more. What's a wine lover to do?
This is where punts, those little cones in the bottom of the decanters come into play. As you consume your wine, the peak of the punt emerges. As the volume of wine goes down, more punt is exposed, further reducing the surface area of the wine. It's an inelegant solution and probably does little to adjust the volume to surface area ratio, but it does make for great marketing speak!