Why the GFSFC cruve decreased rapidly

Hi all,

Here is the GFSFC curve of my NU-refinement and I found the purple curve decreased rapidly. Then I checked the 3D map in Chimera and there was too much noise in this map.

Here are my questions:
What does the “corrected” mean in the curve? How does it work?
Why did the curve drop rapidly? How does it relate to the 3D map?