Pilgrim, I recently stumbled upon the answer. The difference is that the DWCPF is a float adjusted index versus the DWCP is a full capitilazation index both of the Dow Jones Wilshire 4500. In english, it's easier to describe by example: if Company A and Company B each have ten million shares in existence and Company C has 20 million shares then under the full capitilzation index, company C will have twice the weight as A and B and A and B will have the same weight. Under a free float index, the weighting is a function of the number of shares available, so if company A is sitting on 50% of it's stock and company B is sitting on 60% of it's stock, company A will have greater weight. The S-fund is based on the floating index according to the fund sheet at TSP.gov
The DWCPF is the more accurate index to follow. I still don't know the answer to the Yahoo question, bugs the hell out me too.