Honestly, because sight heads and reticle in the case of the NAVY were often contracted by air groups or air support organizations. I get the inference that these are results of pilot experience in combat and their feed back. I have no clue how many reticle for the Mk8 were in use during ww2. I've only recently begun to find pictures from personal collections. And then there was the Mk8 used by the AAF that had the N9 AtA and AtG reticles mounted in it.
The original Mk8 reticle with the hash ladder carried on into Viet Nam being used as the Mk20 in Skyraiders, A26 and other aircraft that used a collimator reflector gunsight for air to ground work.
Context, seems three of them are for air to air work including one as a night fighter reticle. The remaining two for air to ground work based on the preponderance of Mil hash.
This link will get you to the NAVY 1945 manual for glide bombing that instructs you to just how an@l the NAVY was about those Mil hash marks.
http://aviationshoppe.com/wwii-us-navy-glide-bombing-how-to-do-it-a-66.html
