One product is not identical to another. The picture comparison clearly shows the difference in quality of the end product when finished with different machine systems as in the case with these automotive inner doors. The explanation is a easy as the differences are clear:
Robots have a point to point control which forces the machine to move to the programmed points exactly. No consideration is given to the path between the points. This has the effect that the path may have any variation depending on the speed and what mass is being transported. There is no path control whatsoever.
In contrast, a CNC machine controls its path continuously and also observes a so called tolerance margin which can be freely set. This means, the tool of the CNC machine moves within an individually set tolerance margin. If this is being exceeded for any reason, the machine stops automatically and signals with an error message that something outside the norm happening.
Cue word Interpolation: A robot very often contains six turning axis aligned one after the other. For interpolation of a straight path which is the most common and the easiest to control, the CNC machine simply needs to operate all six axes and smooth them in a very complicated way.
The picture shows the result: The path of the robot trimmed part is rough and uneven. The trimmed path of the CNC machine is precise and very smooth and more exact by almost two class sizes. However, the path speeds are nearly treble and the CNC path is being controlled permanently.