Campground Reservations

I just talked to a campground we were going to make a reservation at and this is what I was told. "We are a first come campground. Your reservation saves a spot in our campground for you. However, we do not guaranteed you can fit in the sites that are available when you show up. If you cannot fit in the available sites then you are out of luck."
Your reservation fee is not refundable.

Is this common? I have always had the ability to reserve a site I can fit in. Not necessarily the numbered site I want, but always one I can get fit into.