[MrForbes]

With the 6wd systems, the center wheels are usually lower than the end wheels. This means that four wheels at one end of the robot is acting as a short wheelbase 4wd robot, since the wheels at the other end are off the floor, and the wheelbase length is shorter than the track width it turns easily.

Good tank tread systems have several rollers on each side, and the middle ones are lower than the ends, so it works the same way.