My remark was not correct. HEAD indicates a branch ref whenever available.

Nevertheless, The branch ref file contains contains the head commit for the given branch. And that is what the original text explicitely explained: "whatever SHA-1 value the reference in HEAD points to"

 I understood that you suggested that given history A<-B<-C<-D, D pointed by master, and B pointed by my_branch. If on my_branch you run "git commit" then, you create a commit C' pointing to B, not from D (the head of branch). This is incorrect.