Well, it would be simple if a basic GPS unit could accurately determine its altitude. However, for a variety of reasons, it can't.
WAAS units process additional signal information to improve altitude accuracy. It's my understanding that the iFly (at least the 720...not sure about the 700) is WAAS-capable, but for whatever reason that capability is not currently enabled.
Google "gps altitude accuracy" or "gps altitude error" if you're curious to understand the "whys" of GPS accuracy better. Here's one decent resource, but it boils down to "there's math involved, and approximations, and the earth is not smooth, and so it's hard, see?".