This is normal and expected, due to the distributed nature of git and
the fact that git-bisect operates on the full topology of history and
not just a linear sequence of commits.
Imagine history like:
A---B---C---D
\ /
\ /
\ /
E---F
where B is good and D is bad. Now, when you bisect, there is no way to
know whether, say, E is good or bad and hence the bisect process may
present E as a tree to try.
Now, if B is the 2.6.25 release, then since E branched off before B, it
will have a Makefile that says 2.6.25-rcX. Which is exactly the
behavior you are seeing.
In short, everything looks fine and is behaving as expected.