I also disagree with the “per-capita” notion. New Hampshire has an enormous legislature and a tiny population. California has an enormous population, and a fairly small legislature. One bad legislator in New Hampshire, by this metric, would make NH look as bad as if half of California’s legislature went to prison.
It does seem a strange way to measure, but NH is near the bottom of the list for corruption.
My two cents: Those states near the bottom are probably accurately among the least corrupt, but at the top and in the middle a lot depends on enforcement.
I suspect maybe the political culture in "less corrupt" MN and neighboring "more corrupt" ND and SD isn't that different, but a few bad apples get caught by good enforcement an,d because of their low population, the bad apples make the Dakotas look more corrupt than they really are.