Why does a Mac have a delay when switching between the built-in speaker and headphones?

For example, I'm listening to something playing on my Mac through headphones. I control the volume, and I notice it on my headphones. I unplug the headphones and not until around 3 seconds later will the sound then start to come from the built-in speakers, and that the volume control will actually affect the built-in speaker volume. The same is true the other way around. On Windows, the switch is almost instantaneous, each way. Why?