i'm assuming, from the letter, that the ATO haven't deducted as much as you thought.
it would be your income after deductions but obviously what your actual income came out to is more than the current year estimate

you can check if online (go to your online services, family assistance, family income history (or something like that!))
you can look at your current year estimate, last years estimate, and if fao have your income, it should show it there for the 11/12 year as well