I just tried BOS-DEN nonstop 31 Jan to 7 Feb through the UK† and Canada††. Southwest came in cheapest on both searches. As has been noted, Southwest did not appear in the same search using www.kayak.com.

I was coming from a US IP. I wonder, if I had been coming from a British or Canadian IP, if I would have been redirected to the corresponding site.

†http://www.kayak.co.uk/flights/BOS-DEN/2014-01-31/2014-02-07

††http://www.ca.kayak.com/flights/BOS-DEN/2014-01-31/2014-02-0...