The thing about ash content of flour is that it imparts a dull, or some might say a grayish cast to the crumb structure. It used to be important that bread had a very brilliant, white crumb, but in today's world the average consumer doesn't really look for that anymore, infact, a yellowish/creamy crumb color is more typical today due to unbleached flour. In pizza crusts there is such a small portion of crumb in the crust that crumb color is a moot issue, additionally, with all of the toppings being dragged down over the crumb no one ever really sees the crumb color anyhow. The other thing about the ash content is that while some wheat will typically produce flour with a higher ash content, more commonly, the higher ash content is introduced into the flour through a longer extraction rate (amount of flour milled from a given amount of wheat). Hence a 76% extraction rate would mean that 76-pounds of flour were extracted from 100-pounds of wheat. Typically, the higher the extraction rate, the higher the ash content of the finished flour. This is due to milling the wheat closer to the bran (fiber) portion of the wheat berry. The flour that is extracted this close to the bran contains protein, but not a high quality, gluten forming protein, hence the resulting, high ash content flour will contain a higher protein content than the same wheat milled to a lower extraction rate. This is why we sometimes see higher protein content for flour made from white wheat varieties which don't perform quite the same as a slightly lower protein content flour produced from a hard red wheat variety. Due to the lighter color of the bran in white wheat is is more common for the miller to mill to a slightly higher extraction, to get a better yield while still retaining an acceptably white color. Red wheats, having a darker colored bran, on the other hand are milled to a slightly lower extraction to retain an acceptably white color, hence the protein is more of the high quality, gluten forming protein.
There is still a whole lot more to bran, but this covers most of what is important to us here.
Tom Lehmann/The Dough Doctor