I'm glad you noticed this too. With "a horse", you always get a horse, but with something more abstract like "the discovery of gravity" or "a representation of

I'm glad you noticed this too. With "a horse", you always get a horse, but with something more abstract like "the discovery of gravity" or "a representation of anxiety", it's far more variable between a scene — perhaps people performing an experiment, or someone falling out of a window(!) — or it's simply a large block of text without any people in the image.