It’s hard to be sure from just an image, but it looks like for whatever reason your navmesh is just not generating correctly for the given radius. That margin between wall and green navmesh polygons should equal the radius, so that if you position the ball such that it’s touching the wall, it should be sat right on the edge of the polygon.
I’ve found the editor navigation settings a little confusing and settled on ignoring those under NavigationMesh in favour of adding an entry to the Supported Agents section at the bottom of the Navigation System category. Maybe give that a go instead.
Also, I’m not sure if the generation is inherently conservative, or if it’s possible that the resolution of the nav mesh could cause these issues sometimes. So it might not be a bad idea to set the supported agent radius slightly larger than the actual radius of your sphere.