The weight loss does not occur in a gradual process throughout the body at the same rate. Rather, it happens a bit here, and then a bit there, and then somewhere else. Therefore the loose skin will also happen at irregular intervals. For example, the face, arms, and legs will lose weight at different times, so the loose skin will also appear at different times.
Also, all this is complicated by the fact that even when the skin actually becomes looser, it may not be noticed until a certain point is reached. Therefore, even if the weight loss technically is smooth and gradual, it may not appear that way.