I’ve found another source (‘The Thanage of Fermartyn’, by Rev. William Temple of Forgue, date unknown) which provides a little more information about the Robbs of Pittrichie, who may be related to ‘our’ Robbs in Auchterless. It begins with some information about Alexander Milne of Pittrichie:

He was some time merchant and Dean of Guild of the city of Aberdeen ; was the son of John Milne, for forty-seven years a farmer in Mill of Grandhome, by Ann Duncan, his wife. He died at Pitrichie house in 1869, leaving the estate of Pitrichie to his nephew, George Robb of Pitrichie, the son of James Robb, convener of the Incorporated Trades of Aberdeen, by Barbara Milne, eldest sister of the above-named Alexander Milne of Pitrichie. He was some time a merchant and treasurer of the city of Aberdeen. He was born February 7, 1827, and died at Aberdeen, 6 June, 1876, aged 49. He was succeeded in parts of the lands of Pitrichie and Auchinhuive, including the manor house of Pitrichie, by his brother, Rev. John Robb of Pitrichie, M. A. He was the eldest son of James Robb, and nephew of Alexander Milne of Pitrichie. 

There’s more information about Aberdeen’s Burgesses of Guild, of which Alexander Milne was Dean from 1843-45, here, and a history of the Incorporate Trades of Aberdeen here (though unfortunately it doesn’t mean James Robb by name).