listsinceblock parameter: target-confirmations
As per: https://en.bitcoin.it/wiki/Original_Bitcoin_client/API_calls_list

We have,


  [target-confirmations] intentionally does not affect the list of returned transactions, but only affects the returned 'lastblock' value.


What does this mean? It means the lastblock entry in your returned object is the last block you can see on the blockchain (as it the block that currently has exactly 1 confirmation as indicated by the argument you passed in). The lastblock entry is the only field you can affect by changing the target-confirmations argument.

This means that if you want all transactions with at least 1 confirmation, you need to filter this list. This begs the question, why isn't the target-confirmations argument used to do this filtering?

This is because the way the function works currently allows for a use-case in which you can constantly call listsinceblock with the value of lastblock that was returned in your last call. By doing this, your transactions list will contain transactions that reach X or more confirmations on the blockchain in exactly one of those calls, (where X in this case is 1).