Apologies for what is perhaps a very basic question:

I have always unreflectingly accepted the common distinction between northern ('Calcutta-style') and southern ('Bombay-style') Devanagari. Recently, though, I noticed that some manuscripts mix the two -- for instance, using a 'northern' ṇa but a 'southern' a, or even alternating between the two kinds of ṇa (in the same copyist's hand). Is there any special significance to this -- for example, particular regions and/or historical periods in which the two styles were less distinct? Or should it just be seen as a personal quirk of the scribe (perhaps an itinerant one)?

Thanks in advance for any light on this,
Martin Gansten