The Structure of the Electric Guitar：What are pickups? – Musical Instrument Guide – Yamaha Corporation

Although the way the coil works is quite simple, the sound will change depending on the number of times the coil is wound, or even the way it is wound.
Although winding the coil more will increase the volume of the sound, if wound too much the sound will become muffled. Even after winding the coil hundreds or thousands of times, the sound could still change if the coil is then wound even several tens of times.
Meanwhile, the issue with how the coil is wound concerns the size of the gap in subsequent windings after the first time around. These gaps are incredibly small: as little as 1/100th of a millimeter. Increasing the size of the gap improves the treble. It gets even more complicated-the winding height and even the surface area of the winding (when seen from above) will affect the sound.