Says here that it's because the index is the sum of a city's export "intensity" ("they export more goods to the U.S. as a share of their local economies") and their dependence on exporting to the US. St. John's Irving Oil Refinery exports 80% south plus the seafood and forestry products primarily go to the US as well.
https://businessdatalab.ca/publications/which-canadian-cities-are-most-exposed-to-trumps-tariffs/
Says here that it's because the index is the sum of a city's export "intensity" ("they export more goods to the U.S. as a share of their local economies") and their dependence on exporting to the US. St. John's Irving Oil Refinery exports 80% south plus the seafood and forestry products primarily go to the US as well.