AES-CBC decrypts to wrong output?
They trick you with the IV. The IV must be the same for encryption and decryption so that the encryption has the functionality that is;
$$ m = D_k(IV,E_k(IV,m))$$
To surprise you, they modify the IV according to their desire but you failed to see that. This is the more general case of the Bit-Flipping attack on the CBC mode.
This attack works because the message has only one block and the change on the IV only modifies the first block. Nothing special there.