"APB Reloaded now uses a Bayesian enhanced version of the Glicko rating system to determine the relative skill of players, based on the outcomes of opposed missions. The believed skill of the player is known as that player’s Skill Rating, which is then translated into the player’s Threat Level (shown on the UI), and also used by the matchmaking system to construct matches where the skill of each side is as evenly matched as possible."
From patch 1.5.1
It has then be adjusted over the years such as the 50% score system, readjusting of the bell curve, and likely updates to Glicko 2 or other tweaks.