Presumably because the price you are referring to is an older study. Progressively the various consultancy companies are revising up their long term assumptions and Atlantic is reflecting a conservative view of these forecasts. They typically have a downwards slope in their pricing and Atlantic has reflected that in its pricing.
Back in FFX's Dec 2021 DFS they note a pit optimisation cost of US$666/t with recent [2021] work using US$900/t. LLL's January 2023 MRE used a pit optimisation price of $1,250/t. LLL's June MRE used a pit optimisation price of $1,500/t. These progressively higher pit optimisation prices reflect an expectation by LLL that there will be higher prices. Using higher pit optimisation prices is will have more ore within the economic pit boundaries. It will also increase the life of mine C1/ASIC costs but that is sensible. If you have ore that costs say $1,250/t to extract and process what do you do if the expected price is $1,000/t? - you leave it in the ground. What do you do if you expect the long-run price to be $2,000/t? - you design your operation to extract and process that ore. If prices collapse you redesign what ore you extract. Using a higher pit optimisation cost allows more ore to be economic and is likely to be a factor in LLL large MRE resource upgrades. I suspect if LLL redid their DFS with the current pit optimisation price their C1/ASIC would increase possibly by quite a bit but that's a different issue.
So using pit optimisation estimates as concentrate price estimates, LLL used $1,500/t on 20 June 2023. Atlantic used $1,500/t for its pit optimisation price on 29 June 2023. I think it may be more correct to say in a more historical study LLL used a lower long-term price estimate than Atlantic is using now. Based on pit optimisation parameters both operations are using basically the same pricing expectations.
The better question is why is Atlantic using lower prices than peer's like Global Lithium (GL1)?
From Global Lithium's Scoping study. They are using an average of US$2,500/t across 2026-2032. ALL uses a price below US$2,000 for 2027 to 2032 so they could have used $500/t more and still only been at what some other projects use.
And a recap of what Atlantic used
(20min delay)