Since it's effectively the same anatomical structure, presented in a grayscale image, I think bone density would change the average color of the image regardles

Since it's effectively the same anatomical structure, presented in a grayscale image, I think bone density would change the average color of the image regardless of what level of pixelation was applied. The chance of detection through some other method isn't ruled out though. It could be hitting off of something mundane like the margins on the edge of an image, how centered the torso is in frame, foreign objects in the body from procedures, any random thing that could bias the data can also lead to false positives. The only way to verify would be to use a new data set, ideally from new hospitals, and see if it has similar results.