This screenshot of an Ars Technica journalist with Phil Zimmerman, PGP author (yes, not the same as GPG), is very telling:
https://arstechnica.com/information-technology/2016/12/op-ed...
https://arstechnica.com/information-technology/2016/12/op-ed...
It could be that Phil has a policy of not storing private keys on his iPhone or something. Is that so unusual?
Anyways, maybe you're privy to context that I'm missing.