Why is my jest test coverage lower if I run the test on a single file?

I am adding test coverage to a large file. When I run the coverage report for all my tests, the coverage is at 80.00%. But in order to add more coverage I run the single file in isolation in watch mode to see the coverage change every time I add more tests.

When I do this the coverage instead of being 80.00% drops to 65%.

Does this mean that there are other test files using methods that add coverage?

Does this mean that if myFile.js has a fetchOrders function, but myfile.test.js does NOT test this function, jest will consider that function to be covered if another file named myOtherFiile.js happens to trigger fetchOrders in its myOtherFile.test.js file?