Here I try to smooth a portion of a simple ellipse with extra points added.

The Smooth tool works as intended — the points stay smooth, the curves does not get any bumps or creases.

The Smooth slider however treats the now selected portion of the path as a separate thing altogether, caring about the local smoothness only, making the flush smooth connections to the whole curves into sharp ones.

I now have two creases I never wanted to create (I am smoothing or what?)
Please rethink how it treats partial selections.