It gives the company flexibility. If they didn't have the right reserve to fit the trip they would be glad you called. Sounds like they had the right reserve, so better (cheaper) to pay the reserve guarantee than to pay you extra. Can't blame them on that one. They may not have assigned it yet, again for flexibility. Once assigned that reserve doesn't have to answer the phone anymore so if an earlier trip pops up that reserve is useless. Just guessing here.