Historical events are quite fixed, but if the air conditioner is too noisy to allow sleep, it can be turned off. Deciding that your work place is a real bummer is easier to do if you can go work somewhere else, and much harder to do if you really don't have anywhere else to go. People will tend to focus on those things that are easiest to change. Opinions already have some ambiguity in them and are often obvious targets. Those cognitive elements that resist change the most will stay, and the efforts at reducing dissonance are usually organized around them.
Experiments show that attitudes tend to shift in the direction of the more recent commitment.
The more recent commitment tends to have the higher resistance to change. It's what makes the unique predictions of the theory possible. It provides an organizing point for figuring out how big the dissonance is and how it will most likely be reduced. If people have no hint that commitments might have negative consequences, then they don't cause the inconsistency, and are less likely to feel a need to explain it. But if people are responsible for deliberately doing something that has foreseeable negative consequences, then the strain to explain will be greater.