I have been reading about the 3DES algorithm and I cannot understand one part. In 3DES we do following operation: $$C=E[K_3,D[K_2,E[K_1,P]]].$$ Where $C$ is ciphertext, $P$ is plaintext, $E$ is DES encryption and $D$ is DES decryption.
In the second part we have the decryption with $K_2$. How can we decrypt a cipher with one key ($K_2$) which was encrypted with another one ($K_1$)?