Unexpected result after calling RPC 'listaccounts', 'getaccountaddress', 'getreceivedbyaddress' sequentially
An account can have multiple associated addresses. Presumably you had another address associated with "" beforehand, and received money using that address.

The getaccountaddress RPC call gives a fresh address. If money had been received already on the previous account address, a new one is created and associated with the account.