How do you know it's not correct? You get different numbers because Garmin Connect is recomputing the altitude gain from a map and the track. So you get a different number. It's not supposed to do that with an Edge 800, since it's supposed to just accept the number from anything with a barometric altimeter. But it's been screwed up for a while now with the transition to Modern Garmin Connect, so you never know what it's going to do.