The nature of the center of gravity of a triangle: the ratio of the distance from the center of gravity to the vertex to the distance from the center of gravity to the midpoint of the opposite side is 2: 1.

That is PG/GH=2∶ 1, that is, PG/PH=2∶3.

Your original question about the origin of 1/3 is incomplete. Please understand for yourself according to the origin of 2/3.