DNS filtering. why are there 2 areas to set this up

i noticed that most opendns settings are instructing to put the dns adresses in the WAN tab area.
however there is also the same option in the parental control tab(DNS filtering)

it works when put in the wan area so why is it possible to enable dns filtering also under the parental control tab(without enabling parental control) and even put in different dns adresses.

as far as i can tell the WAN option is leading over the parental control option