Most federal law enforcement agencies have a written entrance examination that an applicant must pass in order to be considered for employment.
Some agencies administer their own test and they maintain a roster of passing scores.
OPM and not the agencies maintains a list of the scores. When an agency is hiring, OPM will provide the agency with a list of the highest passing scores. This system is designed to prevent any nepotism. Once an agency has exhausted the list, if they continue to hire off the same test, OPM will then provide them with the lower passing scores. Therefore, the higher you score on the entrance test, the sooner you will move onto the next phase of the hiring process which is usually the oral interview.