Because the magnetic field or the part of the string being sensed by the pickup is soooo much smaller and more focused with a single coil even microscopic changes in pickup height make a perceivable sonic difference.
Ceramic magnet single coil adjustment height.
Ceramic magnets are made of ceramic surprise.
They rely on support often a foam strip placed underneath in the pickup cavity to push the pickups to the correct height.
By some players the ceramic magnets are given a bad reputation when compared to alnico which is probably due to their association with cheaper instruments.
A fender jaguar s pickups are adjusted very similarly to a jazzmaster.
By adjusting the amount of foam in the cavity you re able to determine the height of the pickup.
You see ceramic is easier to come by than alnico making it a cheaper magnet to make.
For example let s say you ve swapped the stock bridge pickup in your strat for a seymour duncan hot rails a high output single coil sized humbucker.
Humbuckers can run a little closer to the strings gibson recommends 1 16th of an inch for the bridge pickup and 3 32nds of an inch for the neck because their magnets aren t as powerful as single coils.
Setting the pickup height too high can cause interference with your guitar strings 2 setting pickup height too low.
The result is a much more focused magnetic field and a more responsive pickup.
Most traditional single coil pickups only allow for height adjustment of the pickup as a whole but on the mfd pickups the pole pieces are individually adjustable.
As far as pickup height is concerned humbuckers are a whole different animal from single coils.
Your magnetic field floats above the pickup and if your strings are out of the field you won t get a rich full tone.