Interesting results (not surprising though). A 235 profile tires (all other things being equal) is about 2.1% taller than a 225 profile tire. Changing one tooth in a speedo gear will change the ratio by around 3%, so whether that makes sense or not depends on how close the speedo is on the next-larger size tire.
It appears that the guys who have accurate speedos with 235 tires replacing the original 225 tires had an original 2% error in their speedo (reading fast), which was corrected by the larger 235 tires. However, that means that if the same speedo gear was used on the XJs equipped with 235 tires out of the factory, the speedo would be reading dead on, or (and here's the rub...) a little slow. That seems to be a big no-no in the industry (to build a vehicle that might be going ever so slightly faster than the speedo indicates), so my guess is that the 235-equipped vehicles have one less tooth on their speedo drive gear than the 225-equipped vehicles. That would produce an approximately 3% change in the speedo's output (making it read 3% faster than the actual speed).
Charley3's XJ started out with 225s as well, but seems to have been corrected with a change in tire size of over 5%, which is at odds with the other guys' results. If his XJ had come out of the factory with 235's, it would make perfect sense (since the jump from a 235 to a tire with an actual diameter of 29.8" is almost exactly 3%, which would correspond to that one-tooth difference in speedo drive gears).
I guess the only question at this point is do the three vehicles that all were supposed to have been shipped with 225 tires have the same speedo drive gear?