An individual skin cell develops differently from a muscle cell because the cells express different genes.
All the given cells present in the body, be it skin or muscle cells have the same DNA and the same set of genes. However, these cells are able to develop differently from each other due to the differential expression of their genes.
Differential expression of genes means that the different cells which perform different function although have the same set of genes, express different genes at different times and thus, look and function differently.
Skin and muscles cells also express a different set of genes and thus they develop differently from each other.
