The pressure level listed in reloading data is an AVERAGE, some rounds would be higher and some lower. No reloading manual or even factory made ammo will be loaded to the maximum SAAMI pressure for every round, their average pressure will be somewhat lower. There isn't enough uniformity between charges thrown, component construction and firearms to assure the same pressure from round to round so some safety factor must be employed to assure that the SAAMI maximum is never exceeded. This is why hand loads need to be worked up and excessive pressure signs can appear even before reaching maximum book load.
There have been more than one reloader who thought they could guess at how much more powder they could add to the book load to get this magical "maximum pressure" and have wound up with their guns in pieces. What you get for pushing beyond safety limits is generally a very marginal increase in velocity that doesn't kill paper or critters any better than a safe load.