There's a big misconception here.

cbengine simply uses some of the metrics CB provides, such as earnings per sale vs percentage offered in comparison to the product price.

Here's a little bit I snagged from Cbengine's blog about this,

"Keep in mind, refund rates are not an exact science. We simply subtract the payout from the average earned to get a percentage. Occasionally vendors change product prices which will skew return rate percentages. We're hoping in the future Clickbank makes precise return rate data available. "

Here's the link, http://www.cbengine.com/blog/post.cf...k-refund-rates