The details like height, weight and volume are measured on my jugs and are included solely to give an indication of size. They do vary greatly on seemingly identical jugs. This comes down to natural variations in production like mold differences, clay density, shrinkage, glaze thickness etc.
For instance a measurement of 7 jugs of the Mackinlay's #5 acquired from the same source and thus assumed to have come out of the same production batch showed a variety in weight from 525 to 580 grams, which is more than 10%.
In the same way, volume is an approximate. It is not measured all the way to the brim, but likely to a level above what would be practical. And here as well there are variations within what seemingly should be identical jugs.