On the search I just did on Kiwi for a common Southwest route, Kiwi has different results: one non-stop, and at a higher fare, when SWA shows several. Previously I'd noted that Kiwi can show interline international itineraries with a segment on WN and a long layover. I've wondered if such a booking done on Kiwi would be confirmed in real time, or maybe needed an agent's intervention and possibly the fare increasing.