Wellwhat I understand about reroll is
There r 3 tables let's says
A
B
C
A contain (common, rare, epic)
B contain (legendry, mythic, set, spirit, vanity)
C contain (arcane, heroic)

Now let's say u killed boss and got a loot of tableA but it got reroll so while u had a chance to get an item from A (common, rare, epic) but after reroll u got ur loot from table B ( legendry, mythic ,set, spirit, vanity)

Similarly if u r actually geting loot from table B but got re roll than suppose u was getting mythic from table B than u get set from table B(one step upgraded)
Now for table C.
Re roll do happens in table C but only if ur iñitially geting a loot from table C than it will again get re roll and may get same or diff item from table C only.