Your not being unreasonable however in MO. where we live and we also camp in the bodering state of KS frequently one thing we found is that in State Parks only a certain amount of sites were able to be the reserved the rest was set aside for walk in's on a first come first serve basis. I actually had the exact same thing happen to me and when I questioned the ranger about it he explained it. It makes sense to me.
I had the exact same thing happen to us over Memorial Day weekend this year. Our plans changed 2 days before the holiday weekend, I got on the website and where we wanted to go was booked solid. I called into the reservation center and while I was talking to them they had a cancellation so I jumped on it. When we arrived at the park it was only half full. The ranger (a different one of course) told us that the rest of the sites weren't able to be reserved there on a first come first serve basis only. Don't know if that's what your experiencing but it may be.