The first is the percentage. We don't know how many direct mail pieces were originally mailed. Perhaps on the first campaign 50 envelopes were sent. Most advertisers don't understand this statement. Response rate does not equal sales.
Response rate equals responses.
Literally, if a person calls to ask a question in relation to the mailing it is considered a response - even if that person never purchases. Again, technically, I suppose they are right. However, it does not sell to each and every target audience member.