Strange SGID-Bit behavior with 2.2. and 2.4

[Richard Mueller]

Hello almighty kernelgods...

a friend and I discovered some strange diffrence in the beahavior
of the 2.4, 2.2 Kernels.

Following Commands were done on both kernels:
---------------------------------------------
su -
cd /
mkdir test
chmod 2755 test
chown rdd test
su rdd
cd /test
touch a
chgrp users a
chmod 7777 a
cp a b
cp /bin/ls d
chgrp users d
chmod 7777 d
cp d e

These results are on 2.4.6
--------------------------
drwxr-sr-x    2 rdd      root          190 Sep 14 15:17 .
drwxr-xr-x   22 root     root          455 Sep 14 14:59 ..
-rwsrwsrwt    1 rdd      users           0 Sep 14 15:00 a
-rwsr-sr-t    1 rdd      root            0 Sep 14 15:02 b
   -->^<-- What's that?
-rwsrwsrwt    1 rdd      users       46568 Sep 14 15:07 d
-rwxr-xr-t    1 rdd      root        46568 Sep 14 15:08 e

And these on 2.2.16
-------------------
drwxr-sr-x   2 rdd      root          103 Sep 14 15:13 .
drwxr-xr-x  20 root     root          369 Sep 14 15:10 ..
-rwsrwsrwt   1 rdd      users           0 Sep 14 15:12 a
-rwsr-xr-t   1 rdd      root            0 Sep 14 15:12 b
-rwsrwsrwt   1 rdd      users       41552 Sep 14 15:12 d
-rwsr-xr-t   1 rdd      root        41552 Sep 14 15:13 e

It happens both with ext2 and reiserfs.

Can anyone explain this behavior? Would be great.

If this is a damn stupid question, please drop it and kill me. :|

Richard Mueller