Justification can consolidate into a single belief like “I am justified in believing X,” overcoming the actual reasoning that justified the understanding in the first place.
When more understanding is developed, the original justification may no longer be convincing, but the understanding is held anyway, since the justification has been superseded by some memory.
Dissonance frequently occurs when there is a realization that the understanding that was previously justified, and thought to still be justified, is actually in contradiction with some new understanding.