How are there such ridiculously high numbers in the final scoreboard?
The "World" scores are averages across an unknown number of players. However, the scores are not verified for legitimacy before Firaxis(Steam?) collects them for display.
Since it is possible to mod the game, including giving yourself egregiously high numbers of anything you desire, the World's "average" is affected, especially if Firaxis is using a simple mean, instead of the median.
In short: People hacked. Outliers mess up averages.