The pole pieces were staggered to increase or decrease the magnetic field to each string to compensate for the amount of output each particular string would produce. In the earlier days you couldn’t find sets of strings that had a plain 3rd (G string). The 3rd string were wound over the inner core (made of spring steel). The magnet for a particular pole is longer or shorter to help increase or decrease the output, better balancing the volume in the string. Looking at vintage pickups you can see that the B string had the lowest magnet because the string produced the greatest output. The majority of output produced from a string comes from the spring steel used in the inner core.

With today’s modern metallurgy, guitar and bass strings have a better balance and output between each string, so pickups have been made with poles all the same length. It’s easier to manufacturer and takes less time to assemble a bobbin when using one kind of magnet. I like using flat poles when bending strings into another strings magnetic field. I can hear volume changes between strings when using pickups with staggered poles.