Because........telling her "return the check and write cancelled" on the contract only answers the question she asked.
My impression was that she was looking for the answer to the question she asked. I also got the impression that when she said "we will decline the offer today", that she had decided to decline the offer. The question did not appear to me to be open to as much interpretation as is being implied.
As a professional, and a real estate agent, I can tell you there IS more to it than what was asked. Obviously I wasn't the only real estate professional who thought that.