Received outputs are associated to the respective address's account,
but spends (at least, those not generated by the satoshi-client itself) get booked
on the "" account. And "move" may be used to update the inter-account balances.
You can use the 'sendfrom' RPC command to send from an account other than ""
This wiki page explains it:
https://en.bitcoin.it/wiki/Accounts_explained