Not exactly the same, but something similar.

You can try it out at https://bing.com/images/create/

Generated with Bing. Prompt: "Impressive realist painting of a castle on a hillside, contrast of light and dark as the sun streams through the rain".

I honestly think it's just because U turn is easier to say than n turn. Because U is a vowel, it doesn't require the glottal stop that's in n(stop)turn.