13.10 - Software Center prompts password of another user

I have a krb5-ldap user in an "administrators" which is added to sudoers list as follows:

%administrators ALL=(ALL) ALL

Now that works fine for all sudo operations.

But when that user wants to install something in the Ubuntu Software Center, he gets prompted for the password of a local "admin" user.

I have no idea why it would behave that way. How can I fix this?

Update

Also tried to add these users to the sudo groups with pam-groups in /etc/security/group.conf:

*;*;%administrators;Al0000-2400;sudo

The sudo group now shows up correctly in groups, however the software center still asks for the password for another, local user.

The solution is to add adminstrators to the policy kit in /etc/polkit-1/localauthority.conf.d/51-ubuntu-admin.conf:

[Configuration]
AdminIdentities=unix-group:sudo;unix-group:admin;unix-group:administrators