Symmetric Vs Asymmetric
The humbucker pickup was designed to cancel the 50/60Hz cycle hum and external electromagnetic interference.
This hum-canceling method uses 2 coils wound in opposite directions. When external electromagnetic waves reach the pickup they produce an alternating current in both coils, but in opposite directions. If the two coils are connected in the right configuration, those two currents will cancel each other out, thus canceling the hum.
A single coil pickup uses only one coil, so it is more sensitive to ambient hum.
Symmetric humbuckers have two coils, each with the same amount of wounds. Each coil produces the same amount of current at the same time but in opposite directions and these two currents cancel each other out. Asymmetric humbuckers have a different number of coil turns, so the hum canceling effect diminishes slightly and the pickups become less quiet.
Asymmetric humbucker pickups gain small characteristics of a single coil pickup. They have more top and more bottom, but are slightly noisier. This gives more clarity and dynamic to the pickup as opposed to symmetric humbuckers that are more compressed.