Different Trending Promotion Rates for Same Type Items in Same Category. Why?

[amydiz]

What would cause the trending promotion rates inside a category for the same type of item to be different?  I recently listed about a dozen or so pins/brooches into the category for pins/brooches and it seems that all of them have different trending promo rates...  I have 3 brands, Dana Buchman, Napier, and Joyland (which I've listed as unbranded)... could the brand names have anything to do with it?  Any help to understand the trending rates within the same category would be greatly appreciated.