I brought home a new 2015 GMC 2500hd this weekend. SLT, pretty well loaded and GORGEOUS! I love this thing! Traded in a '13 GMC that was an excellent truck, just boring. I never fell in love with...
As for the height difference, as mentioned the same frame/ drivetrain is under the HD's since '11. Two possibilities I can think of: you were not parked in the exact same spot I am guessing so maybe a slight ground difference (2" difference though???), or the bed is a little deeper than the last truck. Otherwise, no idea unless your truck's rear springs had settled that much from the 5ver unless your measurement was from new, not when you traded...