git-checkout question

Hi, 

I was wondering if someone could explain the following behavior.

1. create and switch to branch off master
2. edit a file in the branch
3. checkout master without committing changes in the branch
4. the changes in the branch are automatically applied in working tree 
   in master

I wasn't expecting the changes in the branch to be automatically
moved to master.  Had I committed while in the branch this doesn't
happen.  I'm using git version 1.5.3, the details are below.

Many thanks!

Cheers,
David

[/tmp] mkdir git-test
[/tmp] cd git-test
[/tmp/git-test] git-init 
Initialized empty Git repository in .git/
[/tmp/git-test] # create hello.py
[/tmp/git-test] git-add hello.py 
[/tmp/git-test] git-commit   
Created initial commit 58282ee: Initial commit of git-test.
 1 files changed, 1 insertions(+), 0 deletions(-)
 create mode 100644 hello.py
[/tmp/git-test] cat hello.py 
print "hello from master."
[/tmp/git-test] git-checkout -b test-branch
Switched to a new branch "test-branch"
[/tmp/git-test] # modify hello.py
[/tmp/git-test] cat hello.py 
print "hello from test-branch."
[/tmp/git-test] git-checkout master
M       hello.py
Switched to branch "master"
[/tmp/git-test] cat hello.py 
print "hello from test-branch."
[/tmp/git-test] # hmmm?