as others have said - this is because there's a few extra vertexes in there - one way to fix this is to select the vertexes in that area and do a weld/fuse on the vertexes with a small radius so it doesn't weld the wrong things.
if you did a mirror that will sometimes (usually) leave stray vertices left over. If you turn on vertex mode you might see it. What you are seeing is that vertex having influence on the smoothing.