Bitcoin misconception: all nodes maintain a copy of the same blockchain.

In reality: most nodes will be able to synchronize to the exact same blockchain, but some won't. E.g. pre-segwit nodes don't see the same data as segwit aware nodes, so they can't maintain the same data.

Yet, this won't cause a fork on the network because what matters is not to have the same blockchain. What matters is to agree about the UTXO set state at all times.