Jaunty - installing .deb without password!

[Nigel Ridley]

I'm not sure what or how it happened but I downloaded Opera from their website and then clicked 
on the downloaded file in Dolphin. Dolphin asked if I wanted to install the Opera .deb - I 
clicked 'Install' and it installed it -- without asking for my password!
I checked it's install location and it was installed into usr/bin/; usr/lib and usr/share (plus 
some subdirectories of usr/lib and usr/share).

How did the system allow me to install something without permission?

A couple of days ago I installed another .deb that I downloaded and Dolphin *did* ask for my 
password.

Any ideas?

Blessings,

Nigel