Why has the phrase "from the ousia of the Father" been omitted from the Nicene Creed?

There seems to be a logical sequence in the Nicene Creed of 325: 1. The Son is “begotten from the Father, only-begotten,” 2. Therefore, He is “from the substance of the Father,” 3. Therefore, He is “of one substance with the Father” 4. Therefore, He is “true God from true God.” But the revised creed of 381 omits the phrase “from the substance of the Father.” That seems to break the link between "begotten" in (1) and homoousios in (3). Any idea why and what the implications are?