A hard-to-vary explanation provides specific details that fit together so tightly that it is impossible to change any detail without affecting the whole theory.
A Good Explanation is hard-to-vary whereas a with a Bad Explanation you could vary the details without affecting the explanation.
A bad explanation is unspecific. You can change any or all of the details without destroying it. Examples of bad explanations:
A good explanation is Hard to vary while still accounting for what it purports to account for.