Nettracker vs. Google Analytics

My company has been using NetTracker since 2005, and Google Analytics since mid 2008. One of the measures we track is Unique Visits to the site. The two programs report different numbers for the Unique Visits. Over a 19 month period, NetTracker's Unique Visits average out to 49,500 per month. For the same period, Google averages 43,500 for an average difference of 6000 per month. However, the difference has been as high as 9100 and as low as 3000.

Can anyone explain why there is such a big variance between the two programs? I have not done much work with these programs (the marketing manager asked me to look into this).
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Nenad RajsicCommented:
I have never used NetTracker before but my theory would be that NetTracker counts search engine and robot visits while Google Analytics doesn't

I was selling one of my sites and installed code to publicly provide the number of visits and other data and StatCounter was reporting around 400 more uniques than Google Analytics.

Google bot crawls my news sites daily and I never see those hits in Google analytics (not as a visitor or uniques). then if you imagine how many bots crawl the internet and how many unique IP address each bot has I think that my theory makes sense.

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