Human perception of loudness is logarithmic, not linear, and as such, some volume controls cater to the human ear using logarithmic potentiometers.
In linear potentiometers, the amount of resistance changes in a direct pattern. If you turn or slide it halfway, its resistance will be halfway between its minimum and maximum settings. That's ideal for controlling lights or a fan, but not for audio controls.
Logarithmic potentiometers increase their resistance on a curve. At the halfway point the volume will still be moderate, but it will increase sharply as you keep turning up the volume. This corresponds to how the human ear hears.
Please see the graph below which approximately represents the difference between linear and logarithmic potentiometers:
