Why does fast-forwarding with git fetch reject an ahead branch?

Question

To fast-forward from origin I can do git merge --ff-only origin/BR if BR is checked out, otherwise git fetch . origin/BR:BR (or git fetch origin BR:BR to also update origin/BR).

Using git merge, if BR is ahead of origin, I get "already up-to-date". Using git fetch, I get "rejected (non-fast-forward)". Why is this?