The SCJP is required before you take either exam so I'm not sure how you come to the conclusion that more people come into the SCWCD "cold" than they do with the SCBCD.
I am fully aware the SCJP is a requirement before attaining SCWCD or SCBCD status.
If one follows the SCJP immediatley with the SCWCD (or the SCBCD for that matter) I wouldn't expect them to be "cold" as you stated and was not implying that.
However, as most people tend to go SCJP -> SCWCD -> SCBCD, by the time they begin to study exclusively for the latter it seems to me that the study/exam process would have been well rehearsed. Which may, or may not, contribute to the higher SCBCD scores you suggested. Add that to any on the job experience attained since the previous certification and I can certainly see why SCBCD scores are generally higher than SCWCD scores which you suggested.
I notice from your response signature that you scored higher on the SCJP than on the SCWCD.
What do you attribute that to?
For me, I went the other way scoring higher on the SCWCD than the SCJP. It was 18 months between the exams so I chalked it up to on the job experience as I only actually studied about seven weeks before taking the SCWCD exam.