What you've observed is the decline of the mass-market paperback and the rise of the trade paperback. Mass-market paperbacks are sold like magazines; they havelimited shelf-life and are considered disposable if they don't sell (the vendor, often a supermarket or drug store, tears off the cover to return to the publisher for credit for unsold books, and dumpsters the actual book). Trade paperbacks are sold and marketed more like hardbacks, and are remaindered if they don't sell.
Charlie Stross talks about it a little bit here
http://www.antipope.org/charlie/blog...the-lengt.html