But most Flowers are in the PPC, and the purpose of categories for categories doesn't exactly rule out the 'this thing in this category is also in the cat it's a cat of'. It's a helpful navigation system. Huinesoron (talk) 18:21, June 29, 2013 (UTC)
However, one of the main points of a category tree is that anything in a sub-category must also be in its parent category. Sub-categories aren't "most of the items in this category overlap with this other category". They're "these are still in the parent category, but are more specific to allow easier connection and correlation". Thus, as you said, category trees prove a helpful navigation system, but they will not be as helpful if they are incorrectly divided.
Not all of the Flowers are in the PPC. The Flowers are an entire species, for one thing, so they aren't restricted solely to staffing HQ for the entirety of their existence, and for another, at least one non-PPC organization headed and/or crewed by Flowers is known, the League of Mary Sue Factories. Therefore, while some of the Flowers would fall under "PPC Staff", like the Department Heads and the Secretaries, others, such as the Yarrow and the Forget-Me-Not, would not, which negates the inclusion of all Flowers as PPC Staff members.
And what was that bit about cats you wrote? It was really confusingly worded, and I'm not sure what you meant by it.