One of the tricky bits of using the agricultural production matrix of a civilization as a demographic determinant is that it is highly geographically dependent. Different crops and different domestication-ready animals are available in different parts of the world and this has a massive influence on development patterns. Wet rice agriculture, for example, is simply more efficient than tilled cereal grain cropping, and therefore ever since the domestication of rice there has always been greater population density in the areas suited to rice production than everywhere else on the planet. A fantasy scenario is not bound to organize the continents and their vegetation patterns in the same fashion as Earth, and therefore could unfold a distinctly different pattern than elsewhere. Human abundance could be something as simple as 'they began where the rice was.'