Repro Steps:
1. Install the Terrain Tools Package
2. Create a Terrain Asset in the scene
3. Go to the painting tab on the Terrain object
4. Select Raise or Lower Terrain
5. Set the Brush Strength to 0.001
6. Paint on the Terrain

Actual Results:
A hill will be formed with very obvious ridges.

Expected Results:
A smooth hill would appear.

Note:
Only Reproduces with Terrain Tools installed, doesn't reproduce with the Built In Brush functionality.