It also makes sense because it might not be a good idea to apply the same criteria everywhere. In an app category with 10,000,000 downloads per month, an app pulling 500 downloads a month might be a drop-worthy also-ran that's mostly just directing a few people away from better options & cluttering results. In a category with 1,000 downloads a month, the app with 500 dl/m is likely the best and most-popular app in that category and dropping it would be tantamount to dropping the whole category.