Although the stars are all coeval, because they have different masses, they evolve at different rates. The evolution rate depends on the mass of the star to the second power of the mass, so a 10 solar mass star evolves 100 times faster than a 1 solar mass star and completes its evolution in 100 million years rather than 10 billion years. Cluster members are not all the same mass, and any variation translates into different evolution times which can be seen in the temperature and luminosity of the cluster members---their H-R diagram.