Well yeah, by it's very nature corruption is almost impossible to measure, and the rankings are based on "surveys of businessmen, analysts and residents' views of the prevalence of bribes and theft. The TI defines corruption as abuse of public office for private gain. "
So I guess if you have had some high-profile cases, it is likely it will influence people's views of its prevalence.
My favourite paper at uni was 'State and Corporate Crime' - VERY interesting.