First secret: There are some items that can be obtained that are not listed under any category. You can get them at a small chance on any category. The lower the category, the higher chance they can appear. One of these items I think some people already have (Kingdom Wing). The others - to be discovered!
Edit: When I say lower category, I mean like category C and D are low. S chance has the lowest (so rare you will probably never see it if you choose S tier), then it increases for each category going the other way.