It makes sense though as Camilla is 'only' a Duchess - she's never been given the title Princess of Wales.
Equally the rules of precendence for royalty and for women are relatively well established. Women who hold rank in their own right trump those who married into it.
Most families have their own rules about who greets granny first (or is just my DH's family?!). This is just elevating it to an art form 