In terms of the difference in drop rates: our goal is to make farming energy at (or above) your character level more effective than farming low-level content. Previously, since you could go anywhere to farm essence (and players then naturally gravitated to the lowest levels that made sense for it) the global drop rate was made especially low. Our experiments this year with 'hard level ranges' for event loot, and 'soft level fall-off' for jewels have brought us to this new system... and this new system's intention is to offer a better drop rate for even-level kills that falls off considerably as you out-level the content.
With respect to the Hauntlet content, I believe that the zone scales up to max level - which should mean that you get the highest drop rate. I am not 100% positive about this and I'm not at a place where I can check the actual data. That said, I'll check this evening and if I'm totally wrong about this, I'll amend this response with the corrected info.
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