Quote from Option Trader:
Okay. This email from IB rep (see bold) is what led to the confusion.
Dear Trader,
As your account earns interest each day during the month, the earned interest calculations are booked to a special âAccrued Cashâ sub-account, one for each currency. Every day, the new calculations for accrued interest are added to the cumulative accrued cash balances from the previous day. While this information may be viewed on the daily account statement, it is not actually credited to the account until the first full week of the following month. Once the interest posts to the account, then IB adjusts the "Accrued Cash" sub-account by the amount of interest posted to the account.