Great question — answering as the UphillTech team:
Some airlines enforce more strict rules on their website than they do via other sales channels - especially when it comes to combining different fares and/or flying partner airlines. It is not uncommon for Google Flights to find/offer specific combinations of flights that the airline's website will not display if you were to carry out the exact same search. For example, the airline might decided not to show any flights with particularly long connections (especially >24 hour connections) in their flight search results - where Google Flights does frequently show such results. Some airlines provide Google Flights with a means to link to their website for such a booking - allowing you to book a...
Full answer by Doc: https://travel.stackexchange.com/a/203974 (CC BY-SA 4.0).
Need it done for your business? UphillTech.