There good reasons why there's not total consistency in mapping. Texture maps that are not optimized for a particular model can "fit" but give unacceptable stretching in some areas, as can happen with extreme morphing of the model. Also, folks will create a different UVmap when they want to enlarge a particular area (like the iris) to be able to give it more detail on the texture map. One model can be mapped in any number of ways depending on what a particular user needs. (Thank goodness for UVmapper!)