There is a good article in handloader mag that might explain your results with lower pressure loads. They showed a graph of pressure over time. It showed that peak pressure might occur at the chamber, but thier was also a pressure spike as the bullet was at the muzzle. With max loads, that spike seemed erratic, but at lower pressures, that spike was more reproduceable and more stable.

If you look at alot of the load data people are saying on alot of pages, they are saying they get .5" groups at max loads. I have loaded up three of these RUM, and I almost always get best results at middle range loads like 94gr of H1000 for a 180gr partition, or 88gr of Re25 with 180 tripleshock.