As Nick says, some manufacturers offer different sizes of the same pellet, the idea being that certain sizes will suit certain barrels better. I've never gone into the measuring thing, but many who have came to the conclusion that stated size bore no resemblance to actual size in many cases. The hardness of the pellet will also have an effect on how easy or hard it is to load. And as the manufacturing equipment ages and wears tolerances / sizes might vary?

Having said all that, a couple of years ago, a shooting friend was struggling to get the expected accuracy from his rifle. I let him try some H&N FTT. That previously elusive accuracy was found. When he bought some he found that they were a much tighter fit in the breech, and much harder to load. Now, I can't remember exactly what those stated sizes were, but I think mine were 4.50mm and the ones he had bought were 4.52 or 4.53. He then went ahead and bought some 4.50 and they fitted like mine.