header-args property

[Michael Gauland <mikelygee@gmail.com>]

I'm confused about the use of the header-args property. It works as
expected when I
export a file, but not when I evaluate a code block manually. For example,
consider this file:

    #+PROPERTY: header-args :var x="Hello" :exports both

    #+begin_src shell
      echo X is $x
    #+end_src

When I export, the result of the code block is "X is Hello", but when I
use C-c
C-c to evaluate the block in emacs, I get "X is"--the variable isn't
defined.

In practice, this means I generally define things like :session for each
block,
rather than using a file-level property. Is this behaviour expected? 

Kind regards,

Mike