This wikipedia snippet may help:
http://en.wikipedia.org/wiki/Putonghua

If I'm reading this correctly, the relationship between putonghua and Mandarin has a parallel in Northern India.
Hindi is an artifice, created from one major regional language/dialect, called KhariBoli, which was standardised and imposed as a national language. There are many variants of "Hindi", and even the standardised version is still sometimes called KhariBoli, after the (Delhi-based) variant that forms the foundation of standard Hindi. Perhaps there is a similiar relationship between putonghua and Mandrain?