The margin narrowed, the impairment charge lifted and the cash profit fell as well at National Australia Bank over the June 2016 quarter.NAB put its cash profit for the quarter at A$1.6 billion, or  "three per cent lower than the quarterly average for the March 2016 half year," and also three per cent less than the June 2015 quarterly profit.A lower net interest margin, said to be mainly due to  "higher funding costs " offset any revenue gain from increased lending. Expenses fell one per cent.NAB's rise in the charge for bad debts (by 21 per cent to $228 million) was mostly voluntary, blamed on an "increase in the mining and agriculture collective provision overlay" rather than any surge in specific troubled loans (a theme in peer bank results this period).NAB, on a statutory basis, recorded a whopping $1.7 billion loss over the half year to March 2016, as the bank untangled itself from a debilitating entanglement in UK banking.Its third quarter statutory profit of $1.6 billion (equivalent to, but  "compositionally different" to the quarter's cash profit) leaves the bank notionally trading in the red.