One way that we understand a story is 'finished' is when we get the sense that all the key story elements have been 'used up' in some way, meaning they've been combined in some way that has created a meaningful and satisfying change in their relationships.
For example, in the petunia story, the most important elements turn out to be Irma and her normal prize-winning petunias, the narrator and her mutant monster petunia, and the narrator's desire to beat Irma at all costs.
Even though it's not completely clear what's going on, the final round works as an ending because all the elements have been combined into a new order:
- It seems like the monster petunia has defeated Irma
- The narrator has stolen Irma's petunia stock
- But the narrator's desire to win at all costs brings her to a sticky end because the wild monster petunia comes for her too.
Because the ending is a little vague, you could imagine this story carrying on if the players had had more rounds; but since this is where the story ends, we tend to interpret the ending in that most signifies 'finality'.
Modern storytelling in the English-speaking world is pretty efficient when it comes to combining and recombining elements: we like to see everything used up. (It's Chekhov's gun: if it's on the mantle in Act 1, it needs to be fired in Act 3.)