Does your device have a barometric sensor? I didn't see one listed in the spec's on Wahoo's site. Barometric data will give a better and more accurate info, but it is prone to stuff that can make it seem more inaccurate than other methods used by GPS devices.

If your device isn't using a baro sensor, then position accuracy can really throw off the elevation gain/loss that is being accumulated. IE, when you go over a bridge, map data tends to show the elevation of the terrain below, not the bridge even if it's 100 feet higher. As you ride along the side of a hill, if one moment the position shows you on the low side of your actual path then the next moment it shows you on the high side, then you've just climbed whether you actually did or not.

I'm not saying that you might not be having other issue. But often I think many have an expectation of accuracy and consistency of information that will never be there.