Seem like this follow the same pattern of luck related triggering, i. e, loot rerolling, loot dropping, essence dropping, and locked crate opening. The success rate of good events is bursting. I have experienced 9 of 10 rerolling from killing Jarls with free reroll and lepre (which turn out to be 1/1000 chance of happening.) On the other hand, there are many time that the rerolling rate is much lower than the 40% (especially when I get a 1-hour full set of elixirs.) This behavier seems like a bug to me. The success of an event need to be truly random.

Back to the supergem chance, I would suggest that if you have tens of item being upgraded, complete many of them only when you get some bursting luck... (like what lymsy has gotten)