Webtrends vs Google

Is anyone using Webtrends to track Google Adwords campaigns? Are you seeing any major discrepancy between the number of clicks Google is charging you for and the number of visitors Webtrends identifies as being from Google?

We dont use webtrends but have on some accounts noticed a big difference in clicks on the stats and on the Google reports. Bring to Google attention and they just send you the pre-formatted replies.

Not very helpful at all.

One reason for a disparity is that Google counts multiple clicks by the same person while most statistical programs only count unique visitors.

That is probably not enough to account for a large difference though. Another potential reason is that all of Google's partner sites aren't being identified as Google clicks.

Perhaps a better question would be does anyone have tracking that closely matches the numbers they get from Google?