I agree that it is peculiar given that the data is presented as the number of generations “it would take”. There is absolutely nothing related to access in Scandinavia (and presumably not in a number of other countries as well) that would prevent someone from the lowest social rung from becoming a top earner as a lawyer, doctor or whatever. I can only assume that the data is mislabeled; it must account for more intangible, personal and interpersonal social factors as well, and it must be based on historical data rather than opportunity in the here and now.