Without open disclosure of the methodology of the scoring used in the study, this study should mean nothing more than guess work and should be regarded as nothing noteworthy.
Here is a link with to their FAQs which explain more details. Also they mention there is a detailed sources description document if you download the raw data. http://transparency.org/cpi2012/in_detail
EDIT: D'oh, looks like jlmendezbonini beat me to the punch.