The reason that their rates are equal is because the K constant going this way and the K constant for the reverse reaction are different values. And so the rates are equal even though the concentrations might not be equal because the K's balance the rates out. Now, once again, it's important to understand that entropy at this point is at its highest because entropy defines the most probable state and equilibrium is the most probable state. Now, it's also important to understand that at this point it's dynamic equilibrium. In other words, the reactions are still occurring. X is still being converted to Y and y is still being converted to x.