Hi @80601 ,
That is because, unlike in the robot that you used in the course that used 180 degree laser scanner with 0 - right, 360 - front and 719 - left, the turtlebot in rosject uses a 360 degree laser scanner that gives 0/719 - back, 180 - right, 360 - front and 540 - left.
Refer this post for more info: Incorrect Robot Scan Values in ROS Basics Course - Wall Follower Robot / TurtleBot3
Regards,
Girish