Tracking Affiliate Clicks: Google Analytics vs. Clickbank Analytics vs. Pretty Link

Hey guys,
Here's something I've been searching an answer for all over the web and couldn't find any.
I'm promoting a clickbank product and I'm tracking clicks on my affiliate links in 3 ways:
1) Google Analytics (event tracking)
2) Clickbank Analytics
3) Pretty Link plugin (for cloaking my hoplinks)

The problem is there's a big discrepancy between the methods, each one is displaying completely different numbers.
Pretty link is always showing the highest number of clicks.
Clickbank analytics shows a lot less.
Google analytics shows very few.

Example:
Pretty link: 100 clicks
Clickbank: 50
Google: 10

Why does this happen?
And which one is correct?

One possible reason is the placement of the statistics code. The visitors might not stay on the page long enough for a specific code to load and the visit to register.

Another reason could be that some of those statistics are including spiders in their visit count and some are not. Thus, the discrepancy.

In my view, if it's a clickbank product, then the tracking provided by clickbank would provide the most accurate representation of whats actually going on. The others you mention have too many variables (time on page etc, as mentioned by poster above) that could easily influence results.

netmatrix,
Are you referring to the google analytics tracking code?
I'm guessing you're right about the spiders screwing my statistics, it looks like pretty link can't distinguish between spider hits and human hits. But what about google and clickbank, can they?