I thought you said easy, i only am versed upto linear equations.
No, it makes complete sense that the the average would be closer to the center.
As @Mr.Strange mentioned, imagine each city as a coinflip. If the odds were 50% tribute, and you assigned “heads” as the “win,” then you’d expect about half of the cities to give tribute (half “heads”). As the odds per city lower, the number will skew closer to 0, but only gradually. As the odds per city raise, the number will skew closer to 1 (100%). As is, the value is “set” to 10%, or 20% odds (depending on which assumption you are making regarding city levels); this means the value will skew closer to 0 than even. The peak seems to be around 1-2 for 10%, and ~4 for 20%.