It is not unusual for new guns/barrels to shoot faster than older ones that have seen a lot of hot loads. Wear seems to reduce pressures and velocities for some reason(s). I don't remember where I read it, but one of my loading manuals has an example of a .357 magnum that lost something like 100 to 200 fps off its hot loads during an extensive testing program.
SO, shooting those hot loads a lot may get you slower loads in the future.
As for the actual pressures, I looked at QuickLOAD. It is not easy to tune QuickLOAD for revolvers, and I was not able (with the little effort I have time for right now) to produce the same load vs velocity curve that you show for the Sierra 300 grain bullet. So, the following results are not really definitive. However, QuickLOAD seems to think that 1320 fps is quite possible with a 7.5" barrel (+1.62" cylinder an no gap) with 18.5 grains of 2400. I had to adjust the case volume quite a bit to get the velocities that low, and the resulting pressures were well below SAAMI limits. I did not use the 19.0 grain load because velocity increase with charge increase seems to be dropping off a lot by that point. The loading density for the 18.5 grain load was only 71% to get the velocity to the point that you measured. But, it is usual to have to increase the case capacity of straight-walled revolver cartridges in QuickLOAD by a substantial value to make it match velocities. I don't really know the answer to the question about whether that also makes it match pressure.
Perhaps Unclenick can add something from his superior knowledge of QuickLOAD.