You have to understand there is 2 distincts list of Vertices
The vertices we always use because they are outputted by most plugin
Mesh.Vertices
And the vertices that are used to make the topology
Mesh.TopologyVertices
Most of the times they are hidden but they are more useful because if mesh is welded or not they don’t change. At a point for example on a corner of a face there is just one TopologicalVertex and sometime many Vertices