It has to do with WAAS, wide area augmentation system, in use now in the USA for non-precision approaches, with added vertical guideance.
I don't have one of these new WAAS compatable units in my private airplane, but I wll attempt to ask on another forum about this, and if an answer is forthcoming, will post it here.
However there has been considerable difficulty with some of these WAAS aircraft installations lately and some folks that have the newer WAAS compatable GPS units are not able to use all functions.
It appears to be a software issue, apparently.