Me and Fight have (as always) been looking into legendary drops. I'm desperately hunting for Kridershot and Calamity for my DaemonHunter, and he is desperate for his Fate of the Fell flail for his Crusader.
I did a lot of testing yesterday and can 100% state the following to be true (apologies if you already knew this)...
When you join/create a game (not a Rift, but the actual game), a Loot Table is created for you. This can include everything from greys, whites, blues, yellows, set items and legendary items.
But, there is a limitation, if you get a Rare 2H Crossbow, such as HellRack, that does not mean that ALL 2H Crossbows are in the drop pool for that game.
Generally, the loot table will generate for your current active character, but occasionally it leaks across other classes - usually as whites, blues and yellows, but sometimes as the rarer items.
Yesterday I ran 3 rifts back-to-back in the same game, and got 3 DemonMachine Crossbows, 2 Eye of Etlichs, 4 Tyrael's Might and 2 Hellcat Belts.
I then quit the game to the lobby, and started a new game. I ran a further 3 rifts and got Marauder Spines x 2, 4 x Puzzle Rings and 2 x Special Health Potions (Anti-Wall) as my only legendaries.
It seems pretty naff that the drop system is essentially pre-determined before you actually kill a mob, but what it can mean is that you rely on on several very lucky occurrences to get what you want...
1. The rare item making the initial Loot Table;
2. It's % chance being hit in the game when you proc a legendary;
We do not know what the Loot Table limitations are (size, selection restrictions etc), but if a rare item such as Calamity, with a 1.6% drop chance is present in the table, you need to first proc. a Legendary drop, then proc. a 1H Crossbow selection (if there are others in the current Drop Table), then proc. Calamity at 1.6%, so the chances are ridiculously low.
What is the best way to use this info?
Mildly tedious, but simply remake the game every rift should allow for greater variation in legendary drops.
We do not know if the drop pool is dictated by player or player class (so each DH shares the same drop table), so if someone in your group gets a super-rare, you could keep playing forever and eventually it may drop for you...