why scan right side folders if update only

[optokl]

I am updating from a folder to another folder.
My original folder contains many subfolders with many files.

I tried to speed up scanning by moving one large folder from the origin (left) to elsewhere.

So now, this folder is existing right (from an earlier sync) but not left.
I had assumed its 600000 files would not be scanned now (I am updating, for that it is irrelevant if there are other files on the right side if they are not in conflict with the left (i.e. sitting in different folder names)).

But still FFS scans all 600000 files, now only on the right side.
I would like to understand why.

[Plerry]

Why not simply exclude that particular folder (and all its contents) from the sync
in your sync definition?