I did a patent search on Google, I think these coupons were machine fed into the package. If the cards were a tad to large for the feeder mechanism it would really cause problems on the line. This would explain the isolation of the thin cards to one factory. It was a customer request easily implemented quite late in the production process. Just an Idea.
http://www.google.com/patents?id=IB9...coupon&f=false
Mike