Think of it like this:
Poles emanate a circular field around the pole. It goes up and down the string, and so far to the right and left. So it gets "more" of the string length, but not all of it as you bend the string in and out of the field right to left.
Blades give off a field that doesn't run as far up and down the string, but the cover the entire set of strings right to left, so good volume as the string moves right to left. But the slice of strings is more narrow.
That is by no means entirely correct - but you get the gist...