Different brands of brass will have different thickness and therefore internal volume. That's why you should never take a max charge load developed in one brand of brass and simply interchange to another. If you went from a fairly thin brass with lots of internall volume to a brand with thicker brass you could quickly be over pressure in what you otherwise consider a safe load. Also your accuracy nodes will not likely be the same.
I have never used Quickload so I don't have any experience playing with the case parameters. As with any load data it is just a reference point. Only careful trial and error will safely tell you limits of YOUR chamber, brass, primer, bullet and powder combination. I also believe it is better to run the H2O capacity test with unsized fired brass. The dead primer will plug up the primer hole and the expanded case will be a better representation of your brass and chamber's capacity. If you find your max pressure signs show up a grain or two higher than the Quickload estimate I would assume it's because you have slightly more internal volume than the program calculated from.