I noticed that my Garmin never says my location accuracy is better than 10 feet. I suspect that's a hard stop--no matter how good it is, nothing better than 10 feet will be reported. I decided to check this by plotting the coordinates given from the Garmin device in Google Maps, then Google Earth, and discovered that if any inaccuracy exists, it's within these two programs. In each case, the program showed my location a good 40-50' from where I actually was. On the other hand, plotting those same coordinates within Bing Maps showed the location within 6' of where the Garmin actually was located. The beta version of MapQuest allows you to enter cooridinates, and it was better than Google, but not quite as good as Bing.
I realize that both the Garmin and the map program being used have inaccuracies that will be additive, but I am very surprised at the Google results. I'm curious if anyone here who has plotted their flight paths within Google Earth has seen similar inaccuracies. Your plane could be shown either flying underground or taking off and landing from a virtual airstrip in the sky.