I realize that comparing Awstats and Webalizer to Google Analytics is like comparing apples to oranges, but each of them track at least basic statistics about visitors and pages.

So why are there often very significant differences in their data?

For example, comparing Analytics with Awstats using some numbers from a small site over the past week:

Awstats

• 78 unique visitors
• 205 visits (2.62 visits/visitor)
• 1,072 pages (5.22 pages/visit)

Google Analytics

• 115 unique visitors
• 240 visits (2.08 visits/visitor)
• 1,275 pages (5.31 pages/visit)

They're similar on the number of visits, but page views and uniques are quite different. I'm familiar with discrepancies of a much higher magnitude on some larger sites, showing that this trend scales proportionally upward.

What is the reason behind the different numbers, even when the data is quite trivial like unique visitors and page loads?