Race difficulty does not matter. They give bonus points (way too many in my opinion) to people placing high at bigger races like world marathon majors.
For example Keira D'Amato at Houston set the American record of 2:19:12 but it was worth less ranking points than Emma Bates running 2:24:20 at Chicago because Emma got more bonus points at Chicago than Keira did at Houston. If Chicago keeps only paying out 5 deep international athletes then it makes it the ideal race for world rankings. Any fast international people often drop out once they're not in the money, which is 5th place. 6th/7th/8th/9th/10th still get a ton of bonus points, and it's a flat course, so you can really move up in the world rankings with a result that isn't equivalent to a different race like New York.
For example Ben True was 7th at New York battling with Bekele and Kibiwott Kandie in 2:12:53, it is a tougher course. This result got him 1178 ranking points
Colin Mickow also got 1178 ranking points from Chicago by finishing 6th (only top 5 got paid so a lot of people dropped out and the field was weaker to begin with) in a 2:13:31 on a flatter course. The same course and day that Rupp was able to run a low 2:06 for 2nd place.
I'm just pointing out that their world ranking points are not perfect, they give way too many bonus points to certain races. Even in sprints, people running slow times at 'world athletics label' track meets are surpassing NCAA sprinters running significantly faster, just because it's supposed to be a more competitive race which isn't always the case.