Important reroll/leaderboard questions

I farmed 50k points in the blizzerax map and looted one mythic item but something seems weird; numerous times I looted elite L110 items without rerolling (like I didnt reroll a pink into the elite). What’s weird is that these items didnt reroll into mythics even though I have over 100% reroll/luck. Before this event, in any other map I played, I rerolled literally every single item. My raging set gives 25%, shamrock helm 20%, daily blessing 20%, thrax 30%, greedy ring 20%, amber brew 7%; this adds up to 122% reroll/luck. How would I not reroll every single item I drop? It’s even weirder because the one mythic item I did drop, I rerolled an elite into the mythic, so why didn’t this happen for the rest of the elites I dropped with no reroll?

Another question I have is are there any prizes for leaderboard winners? I couldn’t find anything pertaining to this on the original xmas event post.

Simple. Luck has no effect on getting mythics. (Well actually it does lol)
Judging by what you said it is 100% random beyond elites.
Might even go as far to say Luck does not apply at all or only very very little.

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Mythics are perhaps on a separate loot table this event. Much like vanities in ERD, for example.