This all goes back to the early days of shave geekdom, which may have started as an underground movement in the late '90's. Old methods from the turn of the 20th century were dredged up, studied, and copied as a guide to shaving. Three passes was the standard. And not much later on, some brushes were discovered that were able to hold enough lather for the whole shave without having to whip up more for each pass. Thus, the gold standard for a brush's capacity (although not overall quality) was born.