confusing permission behaviour

[Jens]

dear all,
i was teaching a friend about permissions yesterday when i came upon this
behaviour i couldnt explain...
pls help me understand this as it seems impossible for linux to behave in
this way.

to reproduce as user joe:
cd ~/
mkdir test
cd test
sudo touch file
sudo chown root:root file
sudo chmod og-rwx file

now - this is what is puzzling - as ordinary user joe:
rm -f file

and after asking for permission linux allows user joe to delete a file to
which he has no write permission - let alone read or execute.

since im not at my computer im reconstructing from memory - if this does
not reproduce ill find the exact scenario and write in again - but there
is def. something strange - i did this maybe 5 times in amazement.
thanks, jens