First route: The underlying simile itself is unusual: it discovers a likeness between scenes that would not normally be compared. The horse-and-sitter image illustrates this route.
Second route: The simile is familiar, but an inventive handling of the metaphor creates a rare expression. The verse about the torrent-like movement of camels illustrates this route.
Rarity of the perceived comparison and rarity of the expression’s design are connected, but they are not identical. The two examples must be classified by what actually makes each one distinctive.