Why is impossible to answer. You'd have to be one of Pietta's management people to know the answer to that, and they aren't saying. Some speculate it's the lawyers talking; grossly underloading leaves them lots of room to wiggle out of a product liability suit. Others claim there's some difference between an Italian grain and the US grain.
Suffice it to say, the right answer is: you cannot possibly oveload a percussion revolver. It will not hold enough powder to reach the safety limit of the cylinder; you can overload the chamber in terms of an accurate load, so don't fill it all the way up. The best load is probably somewhere between 20 and 30 grains; you get to try out several loads and see what's best for yourself, which, after all, is a great excuse to go shooting.
Some folks will now chime in and tell you what works best for them; don't assume it'll be best for you, but listen carefully as their experience is not meaningless. Take their advice and try what they suggest, plus a little either side and you'll get the right answer for you.