Please note the following blurb taken directly from AdWords help files:

http://adwords.google.com/support/bin/answer.py?answer=6352

Currently, Google can report only conversions for ads shown on Google and some Google Network sites or products. This means that not all conversions originating from your AdWords ads will be reported. However, your conversion rate, cost-per-conversion, cost-per-transaction, and value / click are adjusted to reflect only those sites from which we can track conversions.

In addition, remember that Google AdWords conversion tracking only reports conversions that occur within 30 days of an ad click. If a customer converts after the 30 days have passed, we don't report that conversion. When viewing conversions for a specified time period, note that conversions are assigned to the date on which the ad click occurs, not the date on which the conversion occurs. Also, we won't be able to report conversion for users who disable cookies.

In other words…Google admits that their PPC reporting numbers can be are way off!