Manifold vacuum is just as it says. Taken from the manifold. Ported vacuum is above the throttle plates. Remember the constriction at the throttle bores creates velocity. cacuum it this pot is governed by the size of the bore. As the throttle plates opemna small slot on the side of the bore in the primaries is less restricted and the value of the port vacuum declines.As you would expect from the engine rpms increasing.