your correct they are very hit and miss, that its why if you care about this metric you have to upload you data so that software can adjust it to correct. IE: Garmin Connect has a altimeter correction built in when you upload it will fix the "weird" readings. Does a nice job of it. Not perfect but still.

I don't have any experience with the Wahoo but I would bet they do the same thing when uploading it to their software.