without exact figures, it's hard to say for sure what happened, but first assumption is that you got supplement for your DS16 last year and not this year

second assumption is that you estimated for fttb (your parenting payment) and overestimated that amount so you got less ftbb each fortnight and a top up now. again, at a guess, i would assume you over estimated more last year than this year.

if you go to your online services you can check this - under family assistance, view family income history
click the drop down and choose 2011/2012
it will show you what you estimated, and then at the very top, what your actual income was
if you click on the hyper links, you will eb able to see your income (not your partner, for privacy reasons - but a calculator will let you take your income off the family income)
you can then compare the actual income to your estimates