Well, I know (just based on experience: I've never seen an article on this) that one time I happened to look at the economy tab in CAII and was looking at corruption %-ages in my cities. I then founded another city, and, since I was over the OCN (actually half of the OCN, since I had no FP) almost all of my cities had a corruption increase (of only 1%, but, if you have many cities over OCN...) So, I assume, once you are over your OCN, corruption increases in all of your cities.
Is this really the way corruption works? By this reasoning, core corruption would increase the more cities you plant and corruption would be less if you reduce your cities. I thought the OCN affects the number of allowed cites before they are completely corrupt. Once over the limit, new cities are just 90% corrupt. I didn't think the number of actual towns affected corruption in the core.
sercer has it right, IIRC, but unless the towns in question are producing a ton of both shields and gold that extra 1% that gets reported in CA2 never really affects anything in game due to the truncation used in the in game calculations.
This is just another example of how CA2 can be inconsistent with actual results due to the nature of the calculations used. I really like the tool but it does have to be employed with some caution. The only numbers you can really trust are the numbers reported in the Domestic Advisor screen. CA 2 is useful for a broad overview but the specific numbers can often be misleading.