[Jailkit-users] Don't understand group permissions

[Caffeine !]

On my system, /etc/group:
www-data:x:33:p
robber:x:1003:

In the jail:
robber:x:1003:
www-data:x:33:p,robber

I don't understand this:

$ whoami
robber

$ ll
total 4.0K
drwxrwxr-- 3 www-data www-data 4.0K Jan  4 16:08 2015/

$ cd 2015/
bash: cd: 2015/: Permission denied

So, despite the fact that user robber is part of group 33, www-data, he still can't cd into 2015/, even though that directory has group permissions of rwx!

The only way robber can access 2015 is if I use my root account and change the permissions of that directory to rwxrwxrwx.

This is even stranger:

$ ls 2015/
ls: cannot access 2015/01: Permission denied
01

The directory structure is 2015/01/, and when I try to list 2015/ it tells me "permission denied" and then goes ahead and lists the directory.

Why are group permissions seemingly ignored?

Why am I told that the directory can't be listed and then it goes ahead and lists it?

This is jailkit 2.17, built from tarball and debianized using debian/rules binary.  The host system is Ubuntu 14.04.1.

Thanks!