I am looking at flights from Boston to Paris and I see that there are two non-stop flights with different travel duration:

![enter image description here][1]

For this itinerary, Air France takes 6h40 while Delta takes 7h04.

Why don't they have the same travel duration? Is it typically due to the type of airplane, the route, or something else?


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Another example:

[![enter image description here][2]][2]


  [1]: https://i.sstatic.net/cWT4m.png
  [2]: https://i.sstatic.net/Wg5HQ.png