In Diablo 3, it is a simple mapping of the higher the level, the better items that can drop. Unfortunately I can't find the Blizzard quote mentioning this, but it's referenced on a number of sites including the one quoted below plus this one.
A good explanation:
Which monsters drop which item tiers depends on the level of the
monster. Level 30 monsters will generally drop tiers of items that can
be used by players level 30 and below. A monster will not drop an from
an item tier that is higher than its level. A level 12 enemy will
never drop a Two-Handed Mace of any quality, since you need to be
level 13 to use a Two-Handed Mace.
Any monster that can drop a given item’s tier and is a higher level
than an item’s required level will be able to drop uncommon, rare,
set, and legendary armor from that given slot. What I mean by that is
if a monster is level 15 and it drops Two-Handed Maces, it will be
able to drop Boneshatter, which is the Legendary level 14 Two-Handed