Hi Tom,
the "speed" you get shown as your unit sits on the desk in your garden is "produced" by the satelites traveling through the sky (the sats are not stationary above the earth). So if your unit doesn't move, the sats are in motion and the unit calculates the different distances of the sats and turns out a "speed" even if you're not moving at all.
You can observe the motion of the sats, if you switch the unit into GPS Status Mode and watch it for a while. Some sats show up, some disappear, but it takes some time to see this.
To get a decent speed fix, the unit has to travel for a minute or two to calculate the "right" speed.
So don't worry, my GARMIN does the same stunt
.
Regards from Germany
Harry