Why the Software Updater asked for password only in the first attemp of installing updates?

Sometimes, the Software Updater asks for a password (normally when you try to install kernel updates).

Every time that I connect to the Internet, the first thing I do is check for updates. And I got 2 updates today.

When I tried to install them, I was asked for the password to install, but I thought it was strange, since it wasn't a kernel update, so I clicked in the cancel button and the update was canceled.

So, I ran again the Software Updater and I got the same programs update, but after pressing to install, it didn't asked for a password and it installed the updates.

So, the question is: Why it only asked in the first time? Is this a bug or anything? Should I be worried about that?

Thank you guys for helping me.

EDIT: I haven't noticed anything strange on syslog and auth.log.