After reporting very strong earnings' growth in 2011, AMP Bank came back to earth last year. Its operating profit of A$62 million in the year to December 2012 was only a fraction above the $61 million result the previous year.The cost of deposits hurt the bottom line. The bank's net interest margin plunged from 1.54 per cent in 2011 to 1.3 per cent in the first half of last year, and then fell further to 1.23 per cent in the December half.Expense management was also an issue. The cost-to-income ratio of 34 per cent was up a little on the previous year.The bank's mortgage book grew 11 per cent, to $12.4 billion, and its deposit book grew 16 per cent, to $8.3 billion. Forty-seven per cent of deposits were sourced from retail, 40 per cent were from AMP superannuation cash and term deposits, and the remaining 13 per cent were from AMP's managed investment fund platforms, including North and Summit.