The Introductory Paragraph.

When I say including efficiency in the healing table is subjective I mean that it implies the notion that people will want to know about it. One could come up with all sorts of statistics about healing, such as maximum and minimum training exp per mp, or HP per second, or MP per second, but no one cares about any of those. The fact that efficiency is there implies a subjective decision on the part of an editor that efficiency was a statistic readers would want to know about more than any of those others.

And as for windmill training varying, every skill's training rate varies with the method used to train it. The method used doesn't change the challenge itself.