Some mining pools send rewards in first transactions in block, this transactions look's like generated, blockexplorer tools can detect list reward adresses, but bitcoin client can't. These transactions get into default "" account.

listtransactions and gettransaction - show only amount bitcoins and transactions id

I think it would be nice if the at the request 'gettransatstion' in the array 'details' were added the field 'address' and is specified correctly 'account'