RandomPointOnSurface sometimes will return wrong position for Hexagonal graph

General Grid graph
#1

I think the RandomPointOnSurface() should return the point which is inside the GridNode, but when I try to pass the random point to GetNearest() it sometimes will return the other GridNode, check the code below, it is about 20% that it will return the wrong point that is outside the GridNode.

            GridNode gNode = ...
            int correctCount = 0;
            int wrongCount = 0;
            for (int i = 0; i < 1000; ++i)
            {
                var kNode = gridGraph.GetNearest(gNode.RandomPointOnSurface()).node as GridNode;
                if (kNode == gNode)
                {
                    correctCount++;
                }
                else
                {
                    wrongCount++;
                }
            }
            Debug.Log(correctCount);
            Debug.Log(wrongCount);