Even the most "identical" set of twins are not "absolutely identical."
The genetic markers for identity may be in the same places,... however, the genetic markers (chemical compounds) do not represent all the characteristics of that compound.
It's like assigning 1's and 0's for the presense or the absense of certain compounds.
In 'identical twins; the ones and zeros will match up.
But this is not a representation of the strength, weakness or other variations in those compounds and or how they will express themselves in the organism's development.