More specifically, multi-city looks at individual fare bucket inventory when quoting a through fare that covers multiple segments vs. looking at "married" segment inventory which is how it is supposed to work (below shows UA quoting an H fare for following flight pair when EF shows there is no actual married segment bucket availability in H on flights). Delta addressed this defect a number of years ago - multi-city searches on delta.com require stopovers (and thus fare breaks) at each stop in the search. Google Flights multi-city search for Delta flights will initially quote fares based on individual bucket availability, but then do a recheck on married segment inventory after you have selected final flight and potentially change the fare quoted if married segment bucket availability differs from individual flight bucket availability.






Here's an example of Google Flights flipping a fare quote on Delta flights (initially quoted a $168 U fare based on individual bucket availability, but then switches to a $259 K fare at final checkout based on actual married segment bucket availability).