John, my guess is it's due to the charter mentioned in the Journal of the Royal Institution of Cornwall, Vol. I. The charter is estimated to have been written between 1159-1175. Per the Journal, the translation is
"Reginald, son of King Henry, Earl of Cornwall, To all his men, French and English, Health. Know ye that I have granted and given to Rohesia de Pomeria, my sister, my Manor of Ridwri, in Cornwall, in free marriage, with all its pertinencies and liberties, in wood, in pastures, in waters, in mills. To hold, to her and her heirs, by hereditary right, of me and my heirs, as freely and quietly and honourably as I ever better and more freely held it. Which, that it may remain ratified and undisturbed, I confirm to her by my charter. These being witnesses: B. * Bishop of Exeter. Roger, Bishop of Worcester. Herbert Fitzherbert. William, my brother. William de Boterell, senior. Richard de Radd. Hugh de Dunstor. William de St. Clare. William of Poitou. Hugh de Valtort.Henry Malo Nepote. Robert de Com____. Baldwin and Richard my grandsons. William de Boterell. Ralph de Ferrers. William de Widiell; and many others."
The book provides notes by Mr. T. Bond: "The Episcopate of Roger, Bishop of Worcester, one of the witnesses to this Charter, extended from 1164 (10 Hen. II) to 1180 (26 Hen. II). The Charter therefore must have been made during that period. The husband of Rohesia de Pomereia was Henry de la Pomerai who is described by Dugdale as a baron by tenure. The site of his barony was Berry Pomerai in Devonshire; and he likewise owned the castle of La Pomerai in the diocese of Baieux in Normandy. Having taken part with John, Earl of Mortaine, afterwards King John, in his rebellion against King Richard I, and having on that occasion seized and garrisoned St. Michael's Mount in Cornwall, he forfeited his lands, and died shortly after the King's return from captivity in Germany. His first wife was Maud de Vetrie, by whom he was father of another Henry de la Pomerai, her son and heir. Rohesia was his second wife, and by her he had issue, Joscelyn de la Pomerai. After his death, Rohesia, his widow, married John Russell of Dorsetshire, and from this match has sprung the present Duke of Bedford."
It continues, "Mr. Stapleton, in his "Rotuli Scaccariae Normanniao," supposes Rohesia to have been daughter of Thomas Bardolf, by Rohesia, daughter of Ralph Halselinus, a baron of Nottinghamshire; which latter Rohesia he considers to have been likewise the mother of Daun or Dado Bardolph; but he was not aware of the existence of Lord Falmouth's charter above transcribed. This charter proves that Rohesia de la Pomerai was sister of Reginald, Earl of Cornwall; and it is likewise proved, from the great Roll of the Exchequer, that she was sister of Daun Bardolph; for, in the 3rd year of King John, John Russell owed 50 marks to the King's Exchequer for marrying the sister of Daun Bardolph. That the wife of John Russell was Rohesia, relict of Henry de la Pomerai, is abundantly proved by contemporary evidence; and, being shown above to have been sister, both of Reginald, Earl of Cornwall, and of Daun Bardolph, she could only stand in that relationship to both these persons by being born of the same mother; for the father of the Earl of Cornwall is known to have been King Hen. I., whilst the father of Daun Bardolph was Thomas Bardolph.
"Adela, daughter of Sir Robert Corbet, Lord of the Manor of Alcestre in Warwickshire, one of the concubines of King Henry I, was mother of Reginald, Earl of Cornwall; and she must therefore have been the mother of Rohesia de la Pomerai. This relationship is alluded to in a passage in Hoveden, who informs us that, upon the conquest of Ireland, King Henry II in 1177 gave the Kingdom of Limeric to Herbert Fitz Herbert and William his brother, brothers of Reginald, Earl of Cornwall, and to Joscelyn de la Pomerai, their nephew; but they refused to accept it, because it was not yet conquered; whereupon it was given to Philip de Braione. Herbert and William Fitz Herbert were sons of Adela Corbet, by Herbert, another husband. Whether Adela was ever married to Thomas Bardolph does not appear.
"After the death of Henry de la Pomerai, Henry his son paid a fine to the King to.have the same seizin as his father had of the Manor of Riduri; but it is difficult to explain why it passed to the elder brother, rather than to the son of Rohesia."