'm not sure what you mean, the points are capped at 570,000, which is a 65.70% chance at a purple, any point beyond the 570,000 will count towards the chance for a second purple.
I guess I assumed that by the drops being independent, each table gets rolled independently and there would be a chance of multiples ( bad ) and require resolving to choose the final drop ( bias )I can verify that the probability for each table when a purple is rolled, is independent of each other.
But maybe you are hinting more along the lines of a single weighted list of all the rewards and a single rolled number retrieving the item from that list. (good)
Again, this is black box for me. So in these scenario's I might be missing a key piece of information, but still find this stuff interesting to understand. And I'm also basing it off the 27 scrolls, 2 dhcb and 3 bucklers our clan received.