Weird behaviour of go-git library when pushing: non-fast-forward update

Question

I am doing the following operations using the go-git/v5 golang library (keeping only the relevant err for brevity

      repo, err := git.PlainOpen(pathToRepo)
      wTree, err := repo.Worktree()
      referenceName := plumbing.ReferenceName(fmt.Sprintf("refs/heads/%s", dstBranch))
      currentBranch, err := repo.Head()
      if currentBranch.Name() != referenceName {
        // checkout new branch
        checkoutOpts := git.CheckoutOptions{
            Branch: referenceName,
            Create: true,
            Keep:   true,
        }
        if err = wTree.Checkout(&checkoutOpts); err != nil {
            return fmt.Errorf("failed to checkout branch %s: %w", dstBranch, err)
        }
      }
      // add files to index
      if _, err = wTree.Add("."); err != nil {
        return fmt.Errorf("failed to add to worktree: %w", err)
      }
      commitOpts := git.CommitOptions{
            All:    true,
            Author: signature,
        }

        if _, err = wTree.Commit(commitMsg, &commitOpts); err != nil {
            return fmt.Errorf("failed to commit: %w", err)
        }

        auth, err := GitAuth(keyPath)

        if err != nil {
            return fmt.Errorf("failed to authenticate to git using key %s: %w", keyPath, err)
        }

        // perform git push
        pushOptions := git.PushOptions{
            RemoteName: "origin",
            Auth:       auth,
        }

        if err = repo.Push(&pushOptions); err != nil {
            return fmt.Errorf("failed git push operation: %w", err)
        }

This is the one that fails

if err = repo.Push(&pushOptions); err != nil

Prior to the above commands, another function modified some files in repoPath.

If after the failure I go manually to repoPath to execute git push (the changes are pushed (!!))

It becomes more interesting by the actual error msg:

failed git push operation: non-fast-forward update: refs/heads/lalabranch

lalabranch is NOT the value of referenceName (which I am actually performing the operations on) but rather of another local (with a corresponding remote) branch.

Why is this happening and why the discrepancy with the actual git commands?