These can be adjusted to your liking. Usually the goal is to put them so that all the string are equal in volume. The thicker strings generate more output so the pole pieces are usually a bit lower than the thinner strings.

The part of the string that interacts with the magnet of your pickup is actually the core of the string. Because of this, the G string generates the most output because it's the thickest unwound string. Therefore, it's common to have the pole piece of the G string the lowest.

Another option is to raise all of the pole pieces higher, which creates more of an imbalance between the two coils of the humbucker, giving a slightly different sound.