Sorry, lomapaseo, my experience on CFM-5A/B/C's is zero. Any of the above could be true, although I'm leaning more to the turbine blade life theory.
I'll also propose that the different fan rotor/stator and the mixed exhaust of the -5C are contributors - i.e. different thrust-to-N1 relationship. Thus simply running the same physical N1 can lead to very different core parameters.
But all of these are merely speculation; I've seen similar effects on other engines.