You say that you have a converged solution–this is likely for a steady RANS solve. Remember the A in RANS stands for averaged. This averaging is done in time to make the problem easier to solve. The downside it, it removes the fluctuations in the flow field and only gives us a time-averaged value. For many problems this is sufficient to provide an engineering representation of the flow field.

An LES on the other hand is a transient simulation that will allow these fluctuations in the flow field to develop. This is could be why you are seeing the unsteadiness in these coefficients. (It could also be diverging. Are the residuals behaving well?) If you were to save all the flow fields from your LES and then average them, it would (to some reasonable approximation) match the steady RANS simulation from above.