The plannerRRTStar class has a GoalReachedFcn that checks if the state of one of the RRT nodes is sufficiently close to the goal to call it "complete". There needs to be some level of tolerance, since in the continuous state space, the robot will never exactly reach the goal. The default GoalReachedFcn is nav.algs.checkIfGoalIsReached, which uses a tolerance of 0.5 meters, and the chart shows the robot reaching within that tolerance.
If you want to use a more precise GoalReachedFcn, you can easily implement your own. The code in nav.algs.checkIfGoalIsReached is very simple, so it'd be easy to copy it and modify the tolerance distance to be smaller.