I doubt that there is an "official" arrest quota, but some people are driven to "excel" at whatever job they have.  They're commonly known as "over-achievers".  If these types (of police) are assigned to a very high crime neighborhood, they can make a real dent in local crime.  However, if they are assigned to a low-crime neighborhood, they can start to feel that they're not "doing their job " and may resort to mentally "upgrading" minor offenses into major ones.  Commonly known as "Making mountains out of mole-hills".