That was my point. If he were to go to the monthly rate, the 9 months would cost more than paying the yearly rate. He is better off paying the total yearly rate and leaving the space unused for 3 months. Many people we know do this and leave their rigs on their site, but some of them bring the rigs home for summer camping up north. The site sits empty, but they know that it will be there for them when they get back and which site they have when they return.
In regards to the OP. I think it was nice of the park manager to allow him to pay monthly and not require the one year payment in full at the beginning of the agreement. But, that's just me. I would not be looking to take advantage of people.
I wonder if this RVer was just confused about the arrangement.