the calculator works out

ftb a
ftb b
paid parental leave
tax
baby bonus

if you qualify for ftbb (so if your partner has a low income), and you take paid parental leave, you don't actually GET that ftbb for the 18 weeks of PPL.
it takes into account a lot of different circumstances and the estimator gives you the option that will give you, over the course of a year, the most cash in hand

i have run the estimator for a lot of different scenarios for DH and I to see what works better. because i get paid mat leave from work, and he is a stay at home parent, there wasn't any circumstance (that was reasonably likely to happen for us) in which PPL works out better for us.