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