I don't know about the 25D and 33D engines, but I've found that it is reasonably common to put out different models that have the same engine with different power ratings, and the only differences are simple adjustments. A good example is the TN55 and the TN65. All you need to do to turn a TN55 into a TN65 is crank up the pressure and flow on the fuel pump. It's just one screw and it takes less than a few minutes. I had heard that this was possible, and today I talked to a very credible source in NC that verified it - and noted that it was one of the reasons that NH is discountinuing the TN55... Too bad I didn't get on the ball earlier, because there were some sweet deals on the discontinued TN55's a little while ago. I saw new ones for $14,300.
I used to teach a class on automotive engineering and IC engines, and it was always fun to show how much easier it is to improve the efficiency and power of diesel engines compared to gasoline engines. In general, if you see two vehicles with the exact same engine model that are rated different, it won't take much to up the power. All you need to do is figure out how to get more fuel into the combustion chamber or up the governor speed.
