It's mostly anecdotal, not measured scientifically. While your thoughts do go down the correct logic path, there clearly is something happening wrt Lil-Gun and SPP. Add to that, that Remington SRP (6 1/2's) were specifically made for Hornet and per Remington's own publicly made comments, have a lower brisance/pressure. More importantly, it is much more prominent with Lil-Gun powder, where pressure curves have been shown to be non-linear near peak pressure. The pressure slowly climbs at peak pressure, and then suddenly spikes as it crossing the threshold of a "safe" load.
There have been (literally) decades of seeing this behavior with Lil-Gun in the Hornet case.
As for QuickLoad, I'd be careful of trusting everything it says. Historically, it never lined up with what was observed in the real world. Close, yes. But never was it ever exact. Granted, I haven't used their software in a few years now (primarily because it ended up being a very expensive "educated guess" when it came to predicting pressures). <shrug> Maybe I'll buy a newer version one day (along with another copy of QuickDesign), but I just haven't found it useful enough in the past to be worth the cost (though I see now that it has come down significantly in price; it used to be $450 a copy, per each software baseline back when I used it all the time).
