Howdy,
This is due to the fact that the physical Sun/Sky is intended to be used with a Linear lighting workflow. Check your camera under the Mental ray tab. An exposure node was plugged in there increasing your Gamma to 2.2 from 1.0.
This is a very interesting topic, Linear workflow. Check out this link and tutorials. Lots of eye opening info here. Specific practices will vary, but the basic idea is the same.
http://www.floze.org/2008/07/six-tuts-on-light-and-shade-part-i.html
Cheers!
Dave

