Sprint Review opportunities

Hello there,
some time ago I participated in the PSPO course and did the PSPO I.
Now that I have gained some experience as PO, I am preparing for the PSPO II.
In the folder from the course I read:

"Sprint Review 'Inspection' reveals opportunities:
[...]
- Ask for a Sprint to implement the demonstrated functionality, alone or with increments from previous Sprints"

I don't understand this sentence. Why would you implement functionality that you already demonstrated in the Review? If it was demonstrated, it should already be implemented, as implementation should be part of the DoD...
And doesn't an increment always include all previous increments?
And why do you ask for an extra Sprint? Isn't it default to start the next Sprint immediately after the last?

Help would be appreciated.

Best,
Ludwig