We don't use anything except the Garmin to determine grade. Why beat yourself up over the inconsistencies? I can determine a trend, comparing different rides, new routes, etc.
Today we did a new variation of a ride we've done several times. It has a good bit of the type of climbing I described, and is around 2,000 feet of climbing. Today's ride had some short and steep and longer and moderate climbing in the new version and the Garmin said 1800 feet of elevation.. It's the only time I have ever felt like I did more climbing than the Garmin said! Probably because it was 53 miles and I just haven't been doing much riding over 40-45 miles this year.
That said, I am the most un-techie person around and I don't have a clue how to work the Garmin. I would actually be happy just to feel like I did some very hard or a lot of climbing and my body will tell me. My DH likes the data, but even he doesn't use his Garmin that often.