This is an effect of the lighting. The picture on the right has much better lighting; it's almost textbook portrait. The compositing messed up the lighting in

This is an effect of the lighting. The picture on the right has much better lighting; it's almost textbook portrait. The compositing messed up the lighting in the one on the left, which makes the features look muted in comparison. My guess would be that the pictures that made up the right-hand composite were more consistent. Either the faces were more consistently in the same place, or they consistently had similar lighting.