There are a few reasons why some 4K images might look better when downscaled to 1080p compared to their original 1080p resolution. One reason is that 4K images have four times as many pixels as 1080p images, which means that when a 4K image is downscaled to 1080p, each pixel in the resulting image has more information to work with. This can result in a more detailed and sharper image compared to a native 1080p image.
Another reason has to do with chroma subsampling. Most consumer and prosumer cameras use chroma subsampling to save on processing and storage. This means that not every pixel in a 4K image has full RGB data, but rather some pixels share color information with their neighbors. When a 4K image is downscaled to 1080p, the chroma subsampling can be reduced or even eliminated, resulting in a more vibrant and colorful image compared to a native 1080p image.
Overall, the increased resolution and improved chroma subsampling of a 4K image can result in a more pleasing image when downscaled to 1080p compared to a native 1080p image.