The caller of the fetch function (Put in whatever forge) is in a context where the reference of the head of the PR is no longer available.
The creator of the fetch function (Get in F3 here) is in a context where the reference associated to the commit it fetches is not yet available.
The F3 fetch function returns the reference to which the commit is associated at the time of the Get so the Put knows which commit to set to the head of the PR it is creating.