I’ve done a variation on this experiment myself. I sit facing forwards by a road, and tap my left knee if a red car goes past, and my right knee for a blue car. When a blue car arrives from the left, I can really feel the difference.
Why does this happen? It’s another assumption about how the world works. Your brain gets ready to respond in the direction towards whatever triggered you. It’s a time-saving measure, just to shave a little time off whatever you’re doing.
In the physical world, it usually works just because. With computers, we need to build that in.