I'm not entirely sure about the "usually", it may or may not be true. But to offer a different view here, I'm from Sweden and cost of living in Sweden is extremely low. If you have an apartment or house outside the capital it costs next to nothing, we have no health insurance costs, no education costs, food is not particularly expensive compared to the rest of the industrialized world and so people usually feel it's OK to offer a lower salary in Sweden on that argument.