As with your father, perhaps your grandmother took on the surname of a later (step?) father. There is a marriage showing on SP in 1895 in Dumbarton for an Elizabeth Murray and a Robert McIntosh that you might want to consider viewing. I think this might be them on the 1901 census:
Robert Mcintosh 39, Coal Man's Labourer b. Edinburgh
Elizabeth Mcintosh 36 b. Dumbarton
Michael Mcintosh 16 b. Dumbarton
Jane Mcintosh 8 b. Dumbarton
Address: 15 Bowie Street, Dumbarton
Not sure about the origins of the Watt name (maybe reputed father

)
Monica