I can't see how the polarising part of a circular polariser can be any less effective than a standard linear polariser (because that's what it is). However, it might be possible that the re-circulificating* bit is somehow reducing the contrast a slightly making people think it's less effective at polarising.
(* I can't think of the right word).
You may have hit it. It could be the contrast difference.
Moma don't take my Kodachrome away!
Oops, Kodak just did!