An obvious reason is that it is thicker. Another one is that it doesn't come in direct contact with the pan - the portion of the egg white that lies nearest the egg yolk sits on top of the second type of albumen, the one that constitutes the entire bottom layer of the cooking egg. A third reason is less known. The inner albumen layer coagulates at a temperature about 5 degrees F higher than does the second type of albumen.
** Asian Recipes