Question about nethack and setgid

[David Demelier]

Hello,

I don't understand how nethack can store the score in 
/usr/local/lib/nethack/logfile.

the real binary is here /usr/local/lib/nethack/nethack

And the mode is

-rwxr-sr-x  1 games  games  1793635 25 Jan 2011 
/usr/local/lib/nethack/nethack

So that means people in games group will run nethack as games user 
(owner of file) but I'm not in games group ! and I can save the file 
located in

/usr/local/lib/nethack/logfile which mode is -rw-rw-r--  games  games

I'm writing a game that will store file too, because I can't make it 
works like nethack I was planning using the setuid mode but I don't like 
much this ..

What am I misunderstanding?

Cheers,

-- 
David Demelier