The typical shell that houses a magnetic pickup has a "wall thickness" of 1/16 inch or 0.063 inch or 1.6 mm. The magnetic structure (bar, polepiece or whatever) cannot come any closer than that to the side of the pickup.

In average playing, the amplitude of the string vibration of a low B string can be as much as 1/4". As much as 0.25 inch or 6 mm.

If the string moves beyond the reach of the magnetic field of the pickup structure (bar, polepiece, etc.) it will not be sensed correctly. The result will be a loss of tonal quality or a compression of the dynamic range (because the largest motions of the string will not be properly sensed).

The magnetic field of any structure does not remain constant up to the edge of the structure but tapers off as a "fringing field", a field that changes strength and orientation as it falls off at the edge of the bar, polepiece, etc. This fringing field does not sense the string motion as accurately as the constant field that exists closer to the middle of the pickup.

For these reasons, it is best to select the length of the pickup shape so that the outer strings are at least 1/4" or 0.25 inch or 6 mm from the outer edge of the pickup shell.

It is also esthetically more pleasing.