I would say majority of people never get to understand their partners more than 60% in whole of their lifetime.
I agree with your statement but my reasoning is that people evolve with time.
What you are in your 20s is NOT what you are in your 30s or 40s.
So understanding between husband and wife is always changing. You need to take a pulse check every six months or year or so and evaluate what has changed, priorities, etc to keep your mutual understanding in the at 60%