Awards are for best in a category, so every photo in a category is open to duels across every sub-category in it. That's how Pixoto works. As well, people can change subcategories without penalty, as often as they want, and there are those who really don't understand the meanings of the subcategories and just pick one at random. That's why subcategories are virtually meaningless here. As long as the category is right, well, that's all that counts.
For some photographs, it does make one pause to consider where to put the image. For example, I have found that architectural black and white photos often do better in Architecture than in the Black and White category. I don't shoot people very often, so I don't know if the same would be true for b&w shots of adults and children.