When making a Fare Shopping for a certain origin-destination, a pricing option might include an itinerary with transfers priced under the same fare (through fare). In this case, if you change the fare of one of the segments, the fare of the other segment will change too since it is the same fare.
Img 1: FS enhanced results for a journey with transfers
Img 2: Original pricing option selected
Img 3: New fare selected for the segment OPO-LIS changed automatically the fare for LIS-CPH segment since OPO-LIS-CPH has a through fare