Much like using small, medium, large, etc. to describe the size of the hat, using fine, extra fine, etc. to describe the grade of panama straw only gives you a reasonably close notion of what you might be getting. It gives you a band, a range, that the hat size, or straw grade might fall into. While this is all well and good if you are looking for something less than specific, it is in no way a precise measurement of the quality of weave you are getting.
Moving on to the number grading system, we start to hit on something a little more precise. The WPI, or Wave (Weave) Per Inch system is achieved by holding a type of box 25mm across, up to certain parts of the panama straw hat, and counting the number of waves or weaves of the straw that are contained by the aperture. The more waves or weaves, the finer the straw, and the higher the quality of the weave. Simple as that!
So, what goes into making a hat of a very high grade of panama straw? What is it about a very high WPI that makes the hat so excellent? The first is that a higher WPI requires a thinner strand of straw, which in turn guarantees a higher grade of straw. A lower grade or straw will never be able to withstand the rigors of being braided into such a fine pattern.
Because the pattern itself is so fine, and the straw so small, this in turn guarantees a better craftsperson. A lesser craftsperson is simply not capable of weaving a very fine straw into such a tight pattern, and so the quality of the craftsperson involved in making the hat is guaranteed by the grade, or perhaps the other way around, if you like chicken and egg arguments.
By contrast, the fine, super fine, etc. system is really only describing bands of ten or twenty total weaves, or waves. This leads to only a vague notion of what the quality the hat you are buying actually is, and reenforces the need for a more precise grading system. Only then will you be able to really understand the craft and the quality of the hat that you are making part of your story.