RobotModel Position [closed]

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This isn't an error that I need fixed, but rather a question on logic. I'm running a group of Turtlebots with the turtlebot_navigation using AMCL. With AMCL, I end up getting a couple of clusters of the Pose Arrays (not terribly surprising). What seems odd though is that the RobotModel is not in the centroid of a convex hull around the Pose Arrays, but rather it's usually the center of the largest cluster. Is the RobotModel position and pose based off of a center of mass, chosen arbitrarily, or just the centroid of the largest cluster? How is the RobotModel position chosen?