Michelangelo’s sixteenth-century Sistine Chapel paintings are currently being restored.
Given that ...
The goal of this restoration is to remove additions made by later artists so that Michelangelo's original work can be seen.
Why is it that ...
They aren't removing additions that were painted by da Volterra?
Evaluate / Goal
We basically need an answer that makes da Volterra's additions different from those of other painters, while relating to the stated goal of trying to uncover Michelangelo's original work.
Maybe da Volterra's additions in some way still count as being part of or contributing to Michelangelo's original work?