How can i understand following sentence?

[richardminh]

We hypothesize that it is easier to optimize the residual mapping than to optimize
the original, unreferenced mapping.

[datatalking]

It sounds like they are optimizing the mapping after it's been "created" rather than optimize before they 'create' the mapping.

Think of it like you clean your data before you work on it vs just throw dirty data in and clean it afterward, here they are doing the optimization after another process rather than before.