And You guessed it! You find low-key lighting on sunny days
Diffused light bounces off rough surfaces like clouds or equipment like umbrellas or silks. Hard light found in low-key lighting scenarios comes directly from sunlight or from spotlights that have specular surfaces inside the structure of the light. Note: shiny surfaces created hard light and rougher surfaces make light more diffuse or soft.
Below find a scene from one of my favorite films: The Third Man Cinematography by Robert Krasker
Low-key lighting can even just consist of something simple like a headlights on a road (or likely a spot light meant to look like headlights)
Below find my favorite modern day Noir moment:
Here's the deal: contrast with low-key lighting doesn't have to be created with art direction--the light creates it.
If you look here to find your 0-10% white values, they are coming from highlights created by harsh light sources. The blacks are created with the shadows that are also created by harsh light. The abrupt change from light to dark is called fast falloff. (conversely the shadows created by soft light have slow falloff.
One of the reasons why high-contrast lighting scenarios look so dark is because the strength of light source. When aiming a spot on a subject's face a cinematographer will have to stop down to avoid overexposing and as a result the shadows get darker.