So that the total cost of the whole journey (Metro+Bus going, Bus+Metro coming back, or vice versa) is the same. E.g.: using a Consortium Travel Card configured for zero fare zone crossings, the total cost of the journey would be as follows:
Outbound trip: Bus+Metro: €0.99 + €0.65 = €1.64
Return trip: Metro+Bus: €0.82 + €0.82 = €1.64