It is possible that somebody else filled the forms out for them and misheard. They may not have been able to read of write. As there is no Smithfield in Northumberland, there are several Drysdales residing in Spittal, which was Berwick-upon-Tweed registration district, in various censuses, I'd say Spittal would be a good line of enquiry to follow. The 1911 census posted before May be a son or relative of the family, I haven't looked to see how he fits in but I would say that as is name is James Wood Drysdale it is likely that he is related to your Margaret and James, and his birth place is also Spittal.