you're going to need 1000s of examples to draw conclusions about it being affected by times and such.
I still think it's completely random chance. like set an item drop rate as 0.5% and so it might on average drop once out of 200 runs. you might get it on the 1st run, or you might not get it even on 400 runs - the chance resets on each run. I do think they lowered the rate of some of the rarer scrolls since the last patch. ;_;