The recommended PSU set by the manufacturer is something that assumes two things:
- A fully loaded ATX system with high-end parts
- The computer running at 100% load.
This is mostly to cover their ass because they can't account for all system configurations (who can?), and you should always plan for worse case anyway. I'm pretty sure the website that does the power supply calculator does actual measurements, among other things.
From what I've seen in most configurations, people won't need in absolution more than 400W of power, even at full load. Even with two video cards (though it depends on the type).