I am using tortoise svn.
When I update the svn trunk directory, it updates to svn branch's head revision.
Does anyone know why this is happening?
When you are updating a working copy it is update to the HEAD revisions unless you specify a specific revision.
svn update brings changes from the repository into your working copy. If no revision is given, it brings your working copy up to date with the HEAD revision.
As revision numbers apply to the whole repository, is it completely normal that you see the most recent revision when updating your trunk.
Unlike most version control systems, Subversion's revision numbers apply to the entire repository tree, not individual files. Each revision number selects an entire tree, a particular state of the repository after some committed change. Another way to think about it is that revision N represents the state of the repository filesystem after the Nth commit.