It's just that the 'Fast' model picks up from a library of existing tracks using a small embedding model.

If you're really curious, you could try the 'Custom LLM' feature - specifically 'Gemini Flash'. Here's the track it created when I gave it the same input.

It titled it, "In the mood for country cowboy-ish music played for someone like John Wick bleeding out on a cold, snow-covered park bench." and here's the justification it came up with, "A weary, grit-laden Dorian lament for a fallen gunslinger, blending the cold isolation of vast reverb with the sharp, intimate pluck of a final, fading heartbeat."

Music link: https://vocaroo.com/1nepiOQ3mrCr