Tbeaux:
Excellent point! Methinks some might've measured alongside the pocket facing, where the slate extends deeper into the pocket than where the "apex" of the pocket drop is. In other words, they might not be placing their ruler dead smack in the middle of the pocket, equidistant from either side of the pocket facings ("splitting the pocket" as a ball would), and measuring from the "earliest point" of the pocket drop, outward towards the beginning of the mouth of the pocket. Not measuring this shelf depth correctly of course, results in skewed results, because the shelf will be calculated to be deeper into the pocket than it really is. If one looks carefully at Roger Long's diagrams in the article, he shows you where to place your ruler to measure each facet of the equation. The double-headed straight arrow in this diagram (the diagram of which is linked here from Roger's article, hope he doesn't mind) shows the correct measurement for shelf depth:
Hope this is helpful!
-Sean