Fwd: Problem: git doesn't update working dir (always) when checkout'ing other branch

Newbie to git, but I've been reading up on the docs and I really love
both the theory and what it let's you do (still confused by some
issues like rebasing and stuff). For now I mostly use it on my own
small projects (where I frequently make branches, merge, cherry-pick
etc), and also push it to  repo.or.cz.

One confusing thing to me is that if I am in a clean state (no updated
files in working dir), and I checkout another branch (or another
treeish), sometimes git doesn't update the working dir, and sometimes
it does. So for example in my own repo I do

~wiki/> git status
no files updated

~wiki/> git checkout trying-new-feature
checked out

~wiki/> git status
no files updated

(this is how it should be obviously - the working dir reflects the
index which is now pointing at trying-new-feature, but then...)

~wiki/> git checkout master
checked out

~wiki/> git status
it then puts me directly into commit mode, with every file having a
ton of differences - because the files are still from
trying-new-feature and the index is pointing at master.

It also seems to do inconsistent things with files that exist in only
one of the branches - ie if switching back the file remains in the
directory but is now an "untracked" file, etc.

Maybe I am completely misunderstanding and there is something I should
do differently. It works out if I do git reset --hard, but it just
seems so uneccessary. (As I said, in all cases the working dir is
completely clean before doing the checkout).

thank you
Stian