Artists are trained in things like perspective, shadows, etc... They often devote a good amount of time drawing people right maybe, study basic principles like mass distribution, but they don't know everything about the underlying principles. For example, I have a painting of a harbor with a few sailboats, it looks great until you realize that sails take the wind from different directions, as if each boat had their own wind. Unless the artist really has a strong focus on realism, sometimes involving collaboration with experts in the field of what they are drawing, you are going to find mistakes like these everywhere.