This kind of thing where a gene is expressed in one individual and not in another "genetically identical" is not surprising to me.
The process of lysogeny in bacteria is well known. Two identical bacteria, infected with the same Lambda phage may either go into a lysogenic state or a lytic state. The key is a "fight" with a repressor. If the balance swings one way the bacterium survives, harboring the lysogenic phage. If the balance swings the other, new virus particles are produced and the bacterium is killed. Why couldn't the same general process happen in identical twins?
Great comment--thanks for the info.