How does the system calculate the turns that a trade route will run for?
I am currently in Renaissance for 5 land routes. Two of them ended in the same turn, but were not created in the same turn. When I looked at the trade routes left, I found that I had :
Edinburgh to Ulundi for 35 turns - 13.9 and 5 gold, 1 and 1 science, 0 and 13 religion
Edinburgh to Stockholm for 16 turns - 15.5 and 7 gold, 1 and 0 science, 0 and 16 religion
Dublin to Stockholm for 5 turns - 10.8 and 7 gold, 1 and 0 science, 13 and 13 religion
Both Sweden and Zulu are on my religion and the religious pressure was mine.
I created two new routes :
Edinburgh - Yekaterinburg for 27 turns - 8.2 and 1 gold, 3 and 0 science, 0 and 27 religion (my pressure onto Russia's different religion)
Edinburgh - Kwasukuza for 24 turns - 11.5 and 3 gold, 1 and 1 science, 9 (Catholic ex Spain, was Zulu's religion before I converted them) and 13 (mine) religion
When the Dublin to Stockholm route expired, I created a new route :
Edinburgh to Timbisha for 26 turns - 7.5 and 1 gold, 3 and 0 science, 0 and 13 (mine) religion
When the Edinburgh to Stockholm route expired, I recreated it for 30 turns.
Why is there a difference in the number of turns, given that I have not made any technologies that would affect it.