Been seeing something weird recently and wondering why. When I search for flights, I often get cases where I get cheaper flights with more search terms. In theory, this shouldn't be the case as more restrictions with more search terms should result in fewer choices.

For example, my most recent search:

Departing: GRR
Outbound routing codes: blank
Outbound extension codes: blank
Outbound date: April 6, 2023

Destination: CPH
Return routing codes: blank
Return extension codes: blank
Return date: April 10, 2023

Cheapest option is a Delta combination of flights for $845.


If I add "alliance skyteam" to both outbound and return extension codes (all other fields the same), I get 49 options that go down to $580. Why is that? I would expect the cheaper itineraries form the narrower search to be the default ones shown on the broader search as they are perfectly reasonable. I have confirmed that they are bookable via bookwithmatrix and Priceline.


In both below screenshots, the left has "alliance skyteam" and the right doesn't.




For the 2nd screenshot, I selected the same exact outbound itinerary, and not even seeing a bunch of the return options from the skyteam search.