when you upload you'll be given 'average speed' (i.e. average speeds of the distance the watch has measured when you turned it on) and 'average moving speed' (basically the average speed of your 'moving' time)
Quite how Garmins measure 'moving' time/how the autopause function works I'm not too sure, as I've had a few times it's reckoned I've stopped moving for about 1mi+n on a race, when I definitely haven't! This may be due to stiles/gates (I don't normally have to wait at these) or, if you're slogging up a particularly arduous hill, your horizontal movement is so limited the Garmin reckons you've stopped!
Either way I tend to ignore 'moving' time.