I'm guessing the the different Garmins use different methods to smooth. It would be interesting to compare the same models on the same ride to check the accuracy.

Lots of riders complain about Garmin's buggy software. For instance, on my 705, if I stop at the side of the road while climbing, and using my wheel sensor so the 705 knows I've stopped, it starts counting up the grade: 5%...6%...7%...8%... What kind of strange grade calculation would work this way!