I tried to figure out why Margaret would take so many years off her age. My first guess was that Robert was a lot younger than her, but not so. Second guess was that there were two Margarets in the family and the first one (b 1872) died, and another was born in 1881. Not so either.
Along the way I found these records for Robert, which fill in the gaps on him:
WWI Draft
Robert Phoenix, b April 11, 1873, naturalized citizen, watchman at Bethlehem Steel, Bethlehem PA. nearest relative: Margaret Phoenix
both at 4 RD Quakertown, Bucks Co
(that should probably be findable at familysearch.org)
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PA birth certificate
Warren Roosevelt Phenix
April 18, 1907 in Haycock Township, Bucks Co
father: Robert Phenix, 36, farmer, of Applebachsville, Pa, born in Antrim Co, Belfast, Ireland
mother: Margaret Waterson, 34, of Applebachsville, born in Belfast, Ireland.
her fifth child, all now living.
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PA Death Certificate
Robert Phoenix
April 17, 1947, in Chester County Hospital, West Chester, Chester Co, PA
his usual residence: Boothwyn, Chester Co, PA
birth: April 11, 1875, Ireland
parents: Matthew Phoenix and Elizabeth __, both born in Ireland
wife, alive: Margaret Phoenix
informant: Margaret Phoenix, of Quakertown PA
buried April 19, 1947, Applebachsville, Bucks Co, PA [but I couldn't have read the town without having his findagrave information.]
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findagrave - same headstone as Margaret, a daughter Edna and son James
https://www.findagrave.com/memorial/17063629/robert-phoenix------------
Civil Registration, Ireland, Registrar's District Annahilt, Lisburn, Co Down
April 11, 1872 in Cargygray
Robert, son of Matthew Phoenix, a weaver of Cargygray, and Isabella Phoenix, formerly Irvine. informant: the father.
https://civilrecords.irishgenealogy.ie/churchrecords/images/birth_returns/births_1872/03252/2191963.pdf----------
So Robert was actually 6 months older than Margaret.