As I understand from newspapers memoirs of both sides*, au pairs had a mother's help type role and were not meant to be treated as servants, their duties being childcare and a little light housework**. In practice, they often were, and are, expected to do almost all the housework, including cooking, though the Wests might employ a cleaner as well, and take major responsibility for children. Since Miranda clearly doesn't need much on the childcare front, it would fit in with the general pattern for Elsa and her predecessors to be expected to basically work as a very badly-paid cook-housekeeper and then be criticised for doing this poorly because they didn't actually have the necessary experience. Claudie has the advantage that she is IIRC a family connection, taken for that reason and not because that's what the family always does about housework, and therefore is situationally much more likely to get the official good experience. Basically, it's the classic servant problem, where the problem doesn't lie with the servants, but employers' expectations of 5* service for 1* wages.

*Here's a recent example from Ireland, based on academic research: http://www.thejournal.ie/au-pair-research-exploitation-uk-ireland-1736486-Oct2014/

**The Wikipedia article describes my understanding of the official duties very accurately.