“Random mutations from the environment. Cosmic rays. These can occur after the zygote has split into the twins. If it happened in a gene relating to features, they could look slightly different.”
Since no identical twins are ever identical, are you saying this always happens? ...and your evidence for that would be what? Or is this more evolutionary conjecture and story-telling?
Hank
No two organisms are identical, even identical twins. They may appear identical, but there can be differences in their DNA. DNA mutations are not always deleterious. They can be neutral or beneficial.
The DNA Polymerases that copy the DNA have a known error rate. Cosmic rays pass through your body all the time.
If they happen to hit your DNA (rare since you and everything else are mostly empty space)they can mutate a base pair.
Slight differences in appearances may be due to the in-utero environment. One twin might might be taking more nutrients than the other when a feature started developing, changing the expression of the genes a little, enough for a subtle change. Things like moles and birthmarks may be due to mutations that change expression when that specific area was developing.
A boss I had had fraternal twins (a boy and a girl). She said the girl was larger when born and bossed her brother around even as infants.