Right. Motor variability, material elasticity, environmental changes, etc. all make that nearly impossible. Think of it like balancing a pole (broom, etc.) vertically on your hand. You can do this because you have real time feedback systems. Close your eyes and you can't. You still have tactile feedback, but that isn't enough. This is what they are catching up to with the machines - gyroscopes, vision/range sensors, pressure sensors, etc. all create feedback loops that allow the robot to respond in near real time. If you notice - there are LOTS of rubber skid marks on the boxes where the robot hit the box and pushed off at a different angle. I'm sure they ran that course many 100's of times. You can see how there are slight deviations in where the rubber feet hit the boxes. Lots of imperfections in the "real world" and the robot must respond and hold the path.