Identical twins were separated at birth and raised by two different families. Years later, one twin was a physically fit member of the cross-country team, and the other twin was overweight with slightly higher-than-normal blood pressure. The differences in these twins could be explained by the fact that

Environment can influence expression of genes

Explanation:

Identical twins almost share everything at birth, and so much of it, that would make you think they would both grow and rarely be differentiated from another but this differentiation can be possible when they share different location. Locations give genes room for expression, basically weather conditions, this could impact in how a child would grow, their size, colour e.t.c

That after a period of time, they would be a notable difference between the twins after sharing different locations.