Why am I asked for the “password for root” when upgrading packages?

Question

I have a fresh install of Ubuntu 12.04. I installed with one user account, then added a second user account using the GUI dialog box method, making myself an Administrator type.

I now use the second user account. When using update manager and it needs to add or remove packages, it asks me for the "password for root" rather than my own password. I've installed and used a lot of Ubuntu systems and never been asked for the root password before. I know how to set a root password so I can work around this situation. sudo works fine and I'm very comfortable on the command line, so I can do everything I need to.

But I want to understand why my account is not allowed to authorise upgrades etc using the GUI.

I have checked the groups and added my account to all that the installuser has.

$ groups
hamish adm cdrom sudo dip plugdev lpadmin sambashare
$ groups installuser
installuser : installuser adm cdrom sudo dip plugdev lpadmin sambashare

Edit

$ id
uid=1001(hamish) gid=1001(hamish) groups=1001(hamish),4(adm),24(cdrom),27(sudo),29(audio),30(dip),46(plugdev),110(lpadmin),114(netdev),126(sambashare),130(vboxusers)
$ id installuser
uid=1000(installuser) gid=1000(installuser) groups=1000(installuser),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),110(lpadmin),126(sambashare)

$ sudo tree /var/lib/polkit-1
/var/lib/polkit-1
└── localauthority
    ├── 10-vendor.d
    │   └── com.ubuntu.desktop.pkla
    ├── 20-org.d
    ├── 30-site.d
    ├── 50-local.d
    └── 90-mandatory.d

$ sudo grep -A 3 Update /var/lib/polkit-1/localauthority/10-vendor.d/com.ubuntu.desktop.pkla
[Update already installed software]
Identity=unix-group:admin;unix-group:sudo
Action=org.debian.apt.upgrade-packages
ResultActive=yes

As you can see from the tree command above, there are no other files under /var/lib/polkit-1 to override com.ubuntu.desktop.pkla. I haven't edited that file either. Does this mean I should be in the admin group as well as the sudo group? Or should being in one group be enough?

What else should I look at?

Edit 2

Just checked and there is no admin group in /etc/group - is that unusual? Should I add that group and then add myself to that group?

(Apparently not - see comment below by @Eric Carvalho)

Edit 3

$ gconftool -g /apps/gksu/sudo-mode
true