When Stats Packages Disagree

Webaliser gives us 300 visits a day, Google says 1700

[tomthumb2000]

I've been using the Webaliser stats package now that comes free with my server and visits have slowly built up to around 300 a day since i started a year and a half ago.

I just got a Google analytics account though and they say we're averaging 1700 visits a day. I know there's different ways to measure all this but what can i think of differences as wide as that?

Does anyone know what's going on?

We've yet to implement adsense sitewide as we're waiting for a recoding, i guess we'll find out then what the truth is.