I think you used standard bus route search with JFK and Heathrow. OpenStreetMap suggests Lisbon since the departure and destination are on different continents,

I think you used standard bus route search with JFK and Heathrow. OpenStreetMap suggests Lisbon since the departure and destination are on different continents, providing a route in close proximity to the destination with access to the sea. If you prefer the flight path, you can visit dev.sitinshade.com/flight [beta].