Blizzard has clarified how the Evoker Legendary drop chance works further, noting that you don't need to do lower difficulty Sarkareths at all to get the full drop chance each week. This has clearly been done to reduce the amount of Sarkareth kills that players of higher difficulties need to do in order to maximize their chances at the Legendary.
This doesn’t apply to regular drops, only the legendary, and you aren’t missing out by doing lower difficulties first.If a character defeats the encounter on Heroic first, then they will have a low chance to get the item and they are covered for the week. Normal and LFR will not offer chances of receiving the item that week.If a character defeats the encounter on Normal first, they will have a very low chance to get the item. Read more on wowhead.com