I will continue my theory:
Kublai Khan also receives +0.3 science and culture per turn for each unit killed.
He has 5.4 sciences and 2.4 cultures per turn
Suppose Ulaanbaatar has grown to 2 citizens, then:
5 (citizens) * 0.5 (science per each citizen) + 2 (geothermal fissure) + 3 (killed barb) * 0.3 = 5.4
5 (citizens) * 0.3 (cultures per each citizen) + 3 (killedbarb) * 0.3 = 2.4
Perhaps a similar modifier works the same for gold.
Kublai Khan also receives +0.3 science and culture per turn for each unit killed.
He has 5.4 sciences and 2.4 cultures per turn
Suppose Ulaanbaatar has grown to 2 citizens, then:
5 (citizens) * 0.5 (science per each citizen) + 2 (geothermal fissure) + 3 (killed barb) * 0.3 = 5.4
5 (citizens) * 0.3 (cultures per each citizen) + 3 (killedbarb) * 0.3 = 2.4
Perhaps a similar modifier works the same for gold.