Since this question was asked a few things related to this have changes (starting with v2.4).
First of all instead of
Unpushed commits (1)
you would now see
Unpushed to origin/master (1)
Which should make this quite a bit more obvious.
Furthermore Magit now supports a push-remote in addition to the upstream branch. The upstream of some feature branch is the branch into which it should eventually be merged (not pushed to). Often the upstream is origin/master. The push-remote is where you actually push to. To learn more about these remotes see the Branching node in the magit info manual.
Configuring two "related branches" for each branch has many advantages. One advantage, which is relevant to this question, is that this makes it possible to show both the commits that have not made it into the mainline yet, as well as the commits which nobody else can see yet, because you haven't pushed them anywhere yet at all. E.g.
Unpushed to origin/master (5) # 5 not in mainline yet
Unpushed to my-fork/feature (2) # 2 of those 5 not at all pushed yet