Unexpected categories

As noted previously here, I'm interested by questions of how best to categorize things.  I'm also intrigued to see how others arrange categories based on what they need to organize.  In this light, I recently returned from a large bookstore and was fascinated to see that there were two shelves designated for "Teen Paranormal Romance".  I wouldn't have thought that this was a sufficiently frequent occurrence to warrant such a volume of material, but there you go.  Similarly, I'd found it pleasantly surprising to visit a store devoted to party goods and to see that they were well stocked with decorative items denoted for 100th birthday parties.  Maybe there are more celebratory centenarians than I'd realized?