Where is the light sensor located on the Surface 2? Seeing the first video only, I notice something odd. What is being depicted on the screen is dark with a light border. The when the user scrolls up, the screen depicts a light background. When the former happens the screen dims, when the latter happens the screen lightens up - which is exactly what happens when the ambient light conditions change. Could it be that the screen is catching its own radiance through the sensor and adjusting accordingly? Certainly, if it is the case that the sensor is on the keyboard (which I suspect is not the case), then this could very well be possible. The only hitch in this theory is that the OP says that the dimming and lightening up the screen occurs regardless of whether or not the auto-dim feature is on or off. If so, then most likely this is a sensor and software problem.
Remember, this is all speculation after a quick glance of the first video only.
That said, I have the auto-dim feature set up on my Surface 2 (I think it does help a bit with the battery life). And, the screen auto-dims as it is supposed to do with variations in the ambient light conditions. But I can't seem to replicate the OP's scenario, which is another question-mark against my speculative theory.