In short: these figures, as we hear them quoted today, are distorted beyond belief, and far more optimistic than they would be if we were using the original measurement criteria from decades past.
It's easy to make a case for why metrics would have to evolve over time: societal changes, technological changes, and the like. But as you'll read (I hope), the amendments made since the 60s have not been made in the interests of accuracy or even sanity. They were made for no apparent reason other than to keep the figures as rosy as possible, and to minimize the amount of money the government would have to shell out for federal debt/interest payments, cost-of-living increases, Social Security payments, what have you.
(If there's one positive to all of this, the article does include a great OM NOM NOM graph...)