If connection was less than 4 hours, cheaper price was likely not due to additive pricing, but getting around married segment inventory availability (it was still pricing on a single fare component, but using individual flight bucket availability instead of married flight segment bucket availability). Example below shows where it is pricing a V fare even though there is no V availability when segments are married. If individual segment inventory and married segment inventory are the same on flights, there won't be any savings. This trick won't work on all airlines (delta.com won't let you use multi-city to book domestic connections less than 4 hours, or international less than 24 hours, to prevent people from getting around married segment availability with multi-city searches).







Google Flights seems to have some sort of deal with Delta where it will do a final married segment availability check on multi-city searches. Below it was initially pricing a $248 SEA-DEN fare with multi-city, but then changed it to $708 SEA-DEN B fare after the final segment was selected and it checked married segment availability. Google Flights does not seem to do this final check on UA and AA multi-city fare searches.