More About 1973 Kellogg's Cards and PSA

Three of the more common cards in the 1973 Kellogg's baseball card set are tough to find in PSA 10 condition.  Wilbur Wood, Jon Matlack and Bobby Murcer are graded 10 at a much lower rate than other common cards in the set.

Most of the cards seem to be easy to find in PSA 10.  They don't seem too expensive either.  It must be the normal processing and the number of factory sets still found unopened.

Does that make sense for Matlack and Murcer since they are at the end of the sheet?  Wilbur Wood doesn't appear on the edge of the sheet.