Builder contracts are entirely different than most normal Purchase & Sale contracts.
IME builders will give you a flat amoumt of $$$ toward closing costs and all of the normal seller closing costs are pushed over to the buyer side. Typically the amount provided toward closing costs by the builder covers the amount a seller normally pays with a small amount left over to cover your closing costs. Sometimes the contribution doesn't even come close to cover the what would be the builders (normal) expense if they were a typical seller. It is also typical that builders do not change there pro-rations after closing - for any reason. Read your contract. What does it say about the builder contribution and the items to be taken out of the builder contribution first? It is usually very specific and they very specifically schedule the charges in a particular order with all of their costs coming first and your costs coming thereafter.
As to your property taxes, how they are handled at closing is detailed in your builders contract. The time to negotiate these items is upfront before you sign the contract. I know it's too late for you now, but future buyers should take note.
