It's probably because the underlying diffusion model is started with grayish noise and can't easily turn that into complete black. At least Stable Diffusion had

It's probably because the underlying diffusion model is started with grayish noise and can't easily turn that into complete black. At least Stable Diffusion had a similar problem. https://www.crosslabs.org/blog/diffusion-with-offset-noise https://www.crosslabs.org/blog/diffusion-with-offset-noise