Purdue Owl lists 10 appositive exercises. I have copied the answers to two of them below. I have setoff the appositives they identify with asterisks since I can't make them bold. My question follows the examples.

The extremely popular American film *Titanic* was widely criticized for its mediocre script.

*The greatest American film ever made*, Citizen Kane, won only one Academy Award.

I understand why "Titanic" (the appositive) is not setoff with commas - it's essential. Why is it that "Citizen Kane" IS setoff with commas? It's not essential, but it's not an appositive either?

Question

Purdue Owl lists 10 appositive exercises. I have copied the answers to two of them below. I have setoff the appositives they identify with asterisks since I can't make them bold. My question follows the examples.

The extremely popular American film *Titanic* was widely criticized for its mediocre script.

*The greatest American film ever made*, Citizen Kane, won only one Academy Award.

I understand why "Titanic" (the appositive) is not setoff with commas - it's essential. Why is it that "Citizen Kane" IS setoff with commas? It's not essential, but it's not an appositive either?