I'm not sure if this is what you mean, but the difference is that $P_i$  is the plaintext block when you really actually decrypt it regularly e.g. in a CBC context; D(C_i) is *just* the decryption of a ciphertext block, so e.g. in CBC mode would still have to be XORed with something before it becomes intelligible :-)

I thought the sentence right before it does explain what that notation means? Or are you asking me to explain why I'm using that notation?