Differences between Omniture & Doubleclick

Hi there,

We are getting different results between our Doubleclick advertising "clicks" and Omniture's "click-throughs".  We are tracking this via Campaign tracking keys and the differences in some cases are significantly different +80%.

I know methodology-wise it can be different, but this is a huge discrepancy. 

Have you come across this before and can you confirm why this is? 

Thanks

Karen