24-120 Nikkor and 15-30 Sigma exposure difference!

Hi all,

A funny thing happened today.............like all good stories this one has a corny start..............I have a D100 and I have been using my 24-120 Nikkor on it for the past few weeks since I got the camera, now I thought it was underexposing too, yes at least 1 stop and sometimes 1.5, no problem of course just adjust in NV5 and alls well, with no blown highlights either, must be how it was designed to work thought I.

But..and heres the funny bit, today I have borrowed a 15-30mm Sigma for trial use, and guess what? the exposures are spot on, all the time, without fail. So I changed back to the Nikkor and immediately got at least one stop difference in exposure, back the the Sigma.....same scene, same time, same everything, but spot on exposure this time.......wierd or what?
It has just got to be something in the "D" chip the lens has embedded.

Any Sigma or Nikon lens engineers reading this care to comment?

Regards David