XRP is a math-based currency just like any other. Why would it need to be a copy of the Bitcoin source to be legit?
I didn't say it did. I said it needs to be mathematically verifiable. Show me how it is and I'll shut up immediately.
Last ledger is the product of a distributed consensus between validators.
Validators will accept a ledger N if and only if there exists a valid transition from previously validated ledger N-1. A valid transition between two ledgers is a set of transactions that are legal as defined by the Ripple protocol. There is no legal transaction in the Ripple protocol that allows to create XRPs, and XRPs can only be moved between accounts or destroyed as transaction fees.
Therefore, a valid transition between two ledgers can only maintain the number of XRPs in circulation or reduce it, and from that property, it derives that the only XRP that can ever be created need to be created in Ledger 0 (or genesis ledger).