You are looking it wrongly.
It should be read as - Successful financial profesionals take many certification exams (including CFA) to learn, re-learn or stay up-to-date in their chosen field.
it should not be read as - People who took certification exams (including CFA) became successful.
The salary numbers you are seeing belong to ‘successful financial professional’. These professionals are also likely to have bachelor’s from top notch schools, MBA from top notch schools, some other certifications e.g. CA, CPA, FRM, CAIA etc.
I believe things will be clear once you read the additional details provided in the survey. Look for age, experience, gender, location, employer etc.
Phil