Each year they recalculate the NPV of decommissioning costs for...

Each year they recalculate the NPV of decommissioning costs for reserves, production profile, future commodity prices,, etc.
So there are lots of moving parts.

The most likely reason for the very large change at 30/6/23 would be a significant reduction in GC21 reserves/production life expectancy (and carrying value?) from 30/6/22 to 30/6/23.

From memory, the 30/6/23 carrying value of GC21 was around $17m (but you can check the 2023 AR).
The carrying value was reduce to nil in the most recent Half Yearly.
This implies that a further significant reduction in CG21 production profile/life expectancy will happen in the 2024 Annual Report.
And a significant increase in the NPV of the
GC21 decommissioning.
The easiest way to view it is that GC21 decommissioning cost are likely to be USD$10m-11m range (my guesstimate) when they happen.
This has to funded from cashflow over time.
In my view: future cashflow form GC21 will not fund GC21 decommissioning, so Lightning and SM71 cashflows will need to fund GC21.

In my view, the GC21 disaster has progressively got worse, and is what scuttled to sale of Otto's assets.
In my view, it is very probable that GC21 now has significant negative value, i.e., Otto would have to pay Talos to take GC21 off their hands.
Talos wouldn't want Otto's GC21, so they would get screwed. Hence they can't sell GC21.
This is a large part of why I sold out.