Corporate pilot was questioned by ATC regarding a descent clearance; the confusion apparently involving a 'pilots discretion' vs; normal descent instruction.

Narrative

Flying along in level flight at FL280; I got an instruction from ATC (Houston Center) to descend and maintain at pilot´s discretion FL240; I read back. According to my VNAV I would be starting descent in approximately 5 minutes. About one minute later I got a hand off to next controller. I checked in and let him know that I was at FL280 and with a clearance to descend at pilot´s discretion to FL240. Then he told me to descend and maintain 9;000 ft. I read back. My confusion was that this was still a pilot´s discretion instruction so I started descent about 4 minutes later. When I started descent; Houston Center came back to me and told me that he gave me a clearance to descend to 9;000 ft 4 minutes ago and that he was showing on his screen that I was just starting descent; he asked for a reason. I told him that I thought this was a pilot´s discretion instruction; after this he told me that no; it was not a pilot´s discretion instruction; it was a clearance to descend to 9;000. I told him that I was sorry about my confusion; then he asked me to expedite descent; which I did.