I'm currently studying about security about Monero, and I found something weird.
I know that the ouputs of Monero transactions are identified by amount and indices (amount 0 for RingCT), and the transactions are using this format.
I recently checked that "6f6f2eea2e549a69ad10246511ccc720193f1d41c9fa2c7aae0e47cb9884d898" transaction with 1,220,517 height of block is the first RingCT transaction in Monero. However, the blockchain data is showing that the output indices of the transaction is not starting with 0 index, rather starting with the index of 2. It seems to be no index 0 output within RingCT transactions.
Furthermore, I found that other transaction is using the 0 index of RingCT output as a mix-in in an input. I wonder what the situation is and what I am missing is.