Earlier quoted context omitted.
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Different priorities. GrapheneOS prioritizes very strong security (inside a very specific model that includes treating the user as an attack vector to be protected against). Fairphone prioritizes repairable hardware (and actually makes their own hardware). On a finite budget (of both money and time and people), prioritizing either one of these will undermine the other.
Fairphone hardware and software is developed/maintained by a Chinese ODM (T2Mobile).