I've found a lot of variation in measuring elevation even among the different Garmin models. I've owned a 510, 520, 800 and 1000 (which is what I currently own). The 800 and 1000 produced similar measurements. The 510 averaged about 10% more than the 800/1000 and the 520 about 10% less. Since I didn't own them concurrently I assume the different firmware versions could have contributed to the differences, but I still have the 510 and when I've had to use it occasionally after forgetting to charge the 1000 it still recorded roughly 10% more elevation than the 1000 over the same course.