No one had a reason for this outcome. It didn't seem to be related to caffeine intake because it didn't matter if the coffee was caffeinated or not!
As a follow-up, there was a look at the data from the Nurses' Health Study and Health Professionals' Follow-up study. These were huge studies with well over 50,000 people involved mainly through validated questionnaires over an 18 year period. When that data was looked at the results were the same as the Finnish study. That is that coffee drinking correlated with a decreased risk in type 2 diabetes, and the more coffee the subjects drank the less the risk. Because of the large number of people in this study, they were better able to look at those who drank only decaffeinated coffee too. Although those people also had a decreased risk, it was not as great as for those who drank regular coffee with caffeine. Also looked at was caffeine intake in general. This showed a graduated decrease in risk too with those taking in the most caffeine on a regular basis showing the greatest decrease in risk.
There are two theories surrounding this phenomenon and both may be part of the reason that we see this decrease. The first is that regularly drinking coffee correlates with other things that are also decreasing the diabetes risk. Those who drink caffeinated coffee are likely to fidget and do other small types of physical activity that would not show up on physical activity level charts. Also if you are drinking coffee, you are less likely to be drinking high carb beverages that might be worse for you. Coffee acts as an appetite suppressant, and those who drink it may have better control over their food choices also. The second theory is that the quinines in coffee are providing this effect. Quinines have been shown to increase insulin sensitivity in rats so that the rats do not need as much insulin to process blood glucose properly.
Either way, should everyone drink a lot of coffee to reduce their risk of diabetes? Well at this point, I do not recommend it. I think each individual should eat and drink the things that make him or her feel the healthiest. If you already have a coffee habit, and it doesn't cause any problems in your life, go ahead and enjoy knowing that you just might be doing yourself some good.
I personally drink between 12 oz. and 32 oz. of caffeinated coffee each day just because I like it.