First you’ll need to understand what pole pieces are and the relationship they have to guitar pickup designs. Pickup poles are magnet posts that run perpendicular to the string windings in the guitar pickup’s coil to serve as a conductor for each string above it. Often you’ll notice one pole piece per string on a single coil is used. Some of today’s designs have one long bar pole piece, or blade, per string. Poles look like small metal circles under each string.
Flat poles are all the same height. The staggered poles have some poles reaching higher for certain strings and some are lower. They have a very non-uniform look. Flat-pole pickups simply give your guitar a better string balance. When the pole pieces are all level then your tone is more balanced. No volume on any string will dominate the others.
Guitar Pickup Designs
Fender Teles until sometime in 1956 had flat-poles. Leo Fender decided that since some of the strings were so far away from the guitar he needed to compensate and make some poles higher. In his mind, it would bring the volume out in the more distant poles for strings sitting higher. Now, moving guitar pickup poles closer to the strings does increase their output-only more windings will do that. However it will make the string louder.
Think about a pickup pole as being a microphone for each string. Think about a singer. What happens as a singer gets farther away from the mic? The volume drops but the mic still has the same output. Placement is important!