Where does the pattern of methylation come from?
Patterns of methylation of DNA are partly inherited from our parents, but the methylation pattern is mostly wiped out from the parents chromosomes at around the 8 cell stage of embryonic development, and a new pattern is added during pregnancy. So, if methylation turns genes OFF, and if methylation can change over our lifetime due to differences in the environment of the DNA, that is a way that identical twins can be different from eachother.
