Priscilla Long explains a misnomer:
Identical twins can be up to 12 percent different. This being the case, the term “identical” has been replaced by “monozygotic.” The difference is caused by epigenetics, which is, in [The Epigenetics Revolution author Nessa Carey’s] words, “the set of modifications to our genetic material that change the ways genes are switched on or off, but which don’t alter the genes themselves.” … Carey encourages us to think of DNA as a script, rather than a template. With a template you stamp out identical gingerbread cookies. With the same script you can make two very different movies. My monozygotic twin and I, whether because of environment, predilection, or epigenetic switches, are two very different movies.