Rush job
SGG liked 2.5%, 5%, and 10%, when Empires first released. This was a rush job.
HotM was not part of original release, so not sure why 1.3%. Never mind, I remember now, CEO Tim loves analytics. Actually probably started with 1.25% and then adjusted upwards while rarity group odds were still unpublished. ( gorram frakking making me have to count heroes in a rarity group for individual hero odds ).
But the largest data collection I know is: