Fleur, the 300ish was your ftb supplement (i'm assuming you had a nil income and got ftbb through the year)
the 2000 would be your supplements. if you underestimated hubby's income, you may have lost part of your supplements to pay back what you were over paid fortnightly. if his income was up over 100k you may have gotten less per child. your youngest princess looks like she was born during the financial year so you wouldn't have gotten teh full amount for her either.

if you're concerned that it doesn't sound like you have received the correct payment, contact FAO