It'll be the mapping that is out. The Garmin mapping for a large area is made up of various charts and normally you can display the chart boundaries on your plotter screen. Garmin literature tells you about possible inaccuracies of the charts relative to each other - and occasionally passing from one chart to another you'll find a stagger in the mapping - and also the possibility of certain patches of a given chart having inaccuracies. My Garmin plotter has a function for off-setting the latitude and longitude of the map relative to the gps fix so if you know where you are, you can move the map to suit. However, their literature warns against relying on this once you move position and I've certainly seen an inaccuracy of the chart but not many meters away it regains its accuracy. I put it down mainly to the digitising accuracy (or inaccuracy!) rather than the original chart accuracy.
As an example, in the detail chart of the Sound of Harris there is a leading line marking a narrow deep(ish) channel and also leading markers on the shore which the chart shows but the leading line and markers on the chart are about 20 meters out of position - wider than the channel! However, less than a 100 meters away there are posts in the water which mark another, and important, deep channel and they are spot on. The harbour is within 400mtrs of these features and the map is off position to the north west by approximately 10mtrs.
I also know that the harbour light on the chart for my local harbour will have you on the reef if you followed it in the dark!
I don't know whether it still exists but there used to be a section on Garmin's web site for reporting mapping errors.
There's a reason for you getting a warning at start-up that the charts are not to be solely relied upon for navigation.
Use the chart plotter but always keep an open mind to what it shows you and what you can see around you. And, always keep an open mind as to what you think you can see around you!
All IMHO, of course.