The Garmin 1000 I use regularly not only accurately records taxiing around the apron, it re-orients as the aircraft turns (track up, so map rotates during turns) while taxiing at walking pace. I'd tech it if it showed map shift while stationary.
There are 2 issues here... The GPS "raw data" and the software processing of that data by the Garmin.
GPS raw data will move around within the error tolerance, caused by inherent errors, receiver quality and signal strength. When stationary it will give +-2-3Kts, and the Track outputs rather random - in short unreliable.
It now comes to the software as to how to handle "smoothing" these errors. "Over smooth" and you get lag and essentially best guess information - as your Garmin is showing. "Under smooth" and you get small shifts, track / GS jumping around. The latter is more accurate if you really want the know what is going on - the former more pleasing if you just like to believe it
