It's more just about how many pokemon are in each tier, and what the overall chances are (
5 k tier has allot of pokemon within it increasing the amount of 5km tier eggs, and allot of them are fairly common, vs 10km with less overall and rarer drop rates, so obviously you get 10km less as a result)
You could work out the chances of each tier based on the amount of pokemon in each + their drop chance etc.