Why does the passport have to have a certain validity after the tour?

Our destination countries have different entry requirements. Some countries demand 3 months validity after exit, some 6 months after entry or after issuing the visa. This can also vary according to the nationality of the participant, so we always calculate with the worst case. Should the tour description indicate 6 months after the tour and your passport is valid 5 ½ months after the tour, we are ready to calculate the exact date for the necessary validity.