So you mean all the transactions are not "finally" confirmed until one block chain becomes the main chain right?
Effectively yes (understand that "finally" is not an absolute thing but the chances of a re-org happening after X confirmations gets harder and harder so that generally once say 6 confirmations have occurred it is considered as pretty much impossible for the transaction to end up later being "undone" by a re-org).
Ah... I see there is a possibility of being wrong despite there exists the only main chain.
Thank you very much.