If a pickup is specifically wound or calibrated for the bridge position it may have more turns of magnet to increase the output for that position. If a pickup is wound hotter for the bridge, it can have more volume and fullness but can sound to dark or fat sounding in the neck position. The string vibration near the bridge is less than the neck position and players often want a better balance between positions. Many guitarists, especially jazz players, have put specifically wound bridge pickups in the neck position for added warmth and fullness.
Pickups that are calibrated for the bridge position can generally have 5% to 40% more turns when using finer gauges of magnet wire for the coils. If two coils in a humbucker are wound with 4,500 turns per coil and the DC resistance could be 7.3 k ohms, the neck position may sound full and warm but the bridge may sound thin and bright. If you put 5,000 turns per coil the pickup could be 8 k ohms and the bridge position could sound fat and full and the neck pickup could sound dark and muddy. Many manufactures wind their pickups the same for each position and when you adjust the amplifier settings for the neck pickup, the bridge pickup would sound thin and less output.
Calibrating pickups for the bridge and neck helps balance the sound when switching from neck, middle and bridge positions. Increasing the magnetic field can increase the output and brightness in a pickup. Over the years players have put larger or stronger magnets in their pickups to increase output and more sustain. I've found that stronger magnets tend to dampen or slow down string vibration and cause the strings to stick to the polepieces when using your vibrato arm.
Note: A tremolo is a mechanical device that varies the volume level, as used in many amplifiers, and vibrato is rapid alteration in pitch or frequency.
WRITTEN ON JUNE 12, 2015, BY