Why does this happen:

Looked at three counters with these results, just want to know why the different numbers.

geo vistors shows like 17
Aw stats 3
Showcase 11
Google 21

Why do the numbers vary and if they are truly counting the unique visits wouldn't they all receive the same info? Or where are the numbers generated from, which are important or that is to say which can be used to gage the real stats?

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Choco777
www.mayatabasco.com