We often take future reservations prior to our setting the rates. The price is not guaranteed until those future rates are set. That is very clearly noted in the confirmation. I would suspect there is a similar disclaimer somewhere in the OP's reservation confirmation. Also, as noted by another poster, taxes can change (and I have never seen them go down) and the park would be in the right to pass those increases along since the tax obligation is generated at each day's stay. (can't pay them in advance).