The ‘revert to original’ and the ‘dismiss all changes’ features in the Onsite Optimizer serve very different purposes.
The ‘revert to original’ feature will remove all changes you've ever published when using the Onsite Optimizer and will revert your webpage back to its original form.
However, the ‘dismiss all changes’ feature will remove any changes that you have made during the current session of using the Onsite Optimizer, meaning all previously published changes will remain in place.