Trust me this is a very good question. When we say “blockchain is immutable” it is the data written on the blockchain (the data structure) immutable. To be accurate, the data placed in a block is protected through block hash, which is chained block-by-block.
In Hyperledger Fabric, the ledger is composed of a blockchain and a world state database. While the blockchain holds the validated transactions, the world state holds the latest state after those the transactions are executed. Therefore, the world state is mutable, and reflects what has been invoked in the transactions. It is the transaction written in the blockchain immutable.
In the example, once you invoke CAR4, the state of CAR4 is changed, but the transaction that “you invoke the chaincode to modify CAR4 to a new value” is immutable. And you cannot deny that you have done this.
Hope this helps.