After mesh,there is a mesh vertex in no point place

[Yes-Jumby]

As the pic shows:

there should be 130 degrees of neck in point cloud ,but after get mesh,the neck be 180 degrees.that is ,50 degrees neck is fake,how to solve this?It is strange，I need help :(
I wonder whether there is graphics knowledge in the pipeline of mesh creating.
Concave Polygon and Convex Polygon,maybe Delaunay triangulation only works for Convex Polygon?