http://news.pickuptrucks.com/2014/07/more-game-playing-now-gm-fesses-up.html Not surprising. Ram will probably next. Or maybe not. Time for another standard for payload ratings.
It's marketing, and anyone who buys marketing hook, line and sinker isn't going to know the difference anyhow.
It's no different than hp and tq numbers being upped by insignificant amount to claim "most". I.e. The 2005 bump in Cummins tq to 610 vs 600, or the Ford 400hp to beat the GM's 397 in 2011.