In the pll of fig 98b an input change of f exactly yields

Question

Question: In the PLL of Fig. 9.8(b), an input change of ?φ exactly yields an output change of ?φ. On the other hand, in the buffer of Fig. 9.8(a), an input change of ?V produces an output change of V/(A0 + 1), where A0 is the open-loop gain of the op amp. How do we explain this difference?