UPG vs. Headcanon: Two Sides of the Same Coin?

There is no functional difference between UPG/ SPG and headcanon/ fanon; it is only because people perceive fandom and religion as distinct entities that they are not allowed to fully coexist. My perception is that this is because people may think that comparing religion to fandom delegitimizes religious practice, but that strikes me as nonsense and I don’t at all agree.