Not necessarily...looking for dinner and looking for lunch will give the CM completely different results. Obviously, I didn't hear the phone call but if you said something like "I want to change my DINNER reservation from LeCellier to Coral Reef on Sept 28" then the CM might have been completely truthful in stating he had no dinner reservations at all that day. He was taking you literally, looking only for dinner time reservations. In this case, he literally would not have seen any availability earlier in the day because their computer systems separate lunch and dinner into separate pools that cannot be viewed simultaneously. I wouldn't say he did anything wrong - he probably searched per your request - but he also didn't go over and above what you requested of him.
The 2nd CM didn't take your request as literally as the 1st did and when she didn't see any dinner hour reservations she probably changed the times and checked for lunchtime reservations to offer as an alternative. Hence, she found availability between 11am and 3:20pm which worked for your party.
Remember, CMs can't read your mind so if you say dinner they're going to be searching the dinner hour inventory and might not realize that you would be fine with a lunch reservation instead. If you are flexible, let them know and most CMs will do their best to fulfill your needs.