My Edge 705 is perfectly happy with reporting a number that is wildly inaccurate. That said, it has the barometric sensor so it shouldn't be subject to the large inaccuracies that GPSs typically have in the altitude department. Even so, as Johnj80 said, it's a difficult thing for a GPS to determine accurately.

That said, the grade display only works when it's actively recording data points (when recording is on rather than off, an odd restriction, but whatever) and occasionally it stops reporting the grade entirely until the entire thing is shut down and restarted.

I'm sort of surprised that nobody makes a grade display that mounts to your top bar, measures it's orientation with sensors like a smartphone has and is calibrated simply by riding your bike around in a few circles on a relatively flat parking lot. Wouldn't be difficult to make and could easily be quite accurate.