From Frank Herbert's original series, some suggestions why this might be the case -
- In Heretics of Dune, Lucilla is described as resembling his biological mother, and Lucilla resembles Darwi Ordrade (an Atreides) who apparently resembles Lady Jessica
- He is from Geidi Prime originally
- He is employed in the breeding programs of Leto II and the Bene Gesserit repeatedly.
- The Bene Gesserit program valued the Harkonnen genetic line, while Leto II describes Idaho as more Atreides than the Atreides (and the Atreides have Harkonnen blood via Jessica)
- He has the black hair and green eyes that Paul Atreides has