Unit error - a mile is not the same as a kilometer

In the watch, Connect or both? In Connect check the measurement units in user settings; Metric, Statute, Statute UK.  In the watch check distance and also pace/speed units in settings>format>units.

I know how to set everything to metric - it's been set that way for years and still is now.  The problem was that it was showing statute numbers for metric data fields.  I don't think I could even run as slow as 8:32 per km, I was definitely travelling a lot faster than that.  I've run the same course about 100 times, I know how far it is.  3.3 km measured on a map, 2.1 km reported, QED. 

Since you have no map for your activity, is it possible that you never got a GPS lock (or that GPS was disabled)? For activities with no GPS, the watch will try to estimate speed via the accelerometer, and it probably won't be very accurate. It could just be a coincidence that the watch happened to record 2.1 km instead of 3.3 km. (Note that 3.3 km is actually 2.05052 miles.) Did the watch actually record the *exact* equivalent distance as if it were miles? i.e. if the real distance was really exactly 3.3 km (or close to it), did the watch actually report roughly 2.05 km? Or was it just kind of close (like 2.1 km)?

I don't think it's possible that the watch is literally recording "miles instead of kilometres" (or otherwise confusing miles with km or vice versa), since the internal base distance unit is metres. (You can see this if you look into the activity FIT file specification, for example, or if you develop 3rd party apps for Garmin devices.) Every time the watch, Connect app, or Connect website displays pace or distance in km or miles, it's converting from internal data which is stored in m or m/s. IOW, km/miles is just a display format, and the internal algorithms and data recording process should be unaffected by the choice of display formats.

Has this happened more than once?

This is quite likely what happened - I did think of this yesterday and gave it some time after my warm-up for GPS to connect but now I remember only seeing a red bar, not green like it did just now when I stepped out the front door.  Never happened before so this hasn't previously been significant.

Thanks for the info. 

No worries!

I will add that in my experience, it’s not enough to wait for the green GPS ring, especially if you’re around tall buildings. Garmins often show the green GPS ring before they actually have a solid fix — you can see this by the fact that even if you’re standing still your pace may be fluctuating wildly (and the map page shows that you’re moving around). If you start your activity at this point, you may have crazy-high speeds and too much distance recorded for the first 30 seconds or so of your activity. (It will also be obvious from the GPS track, which will show that you ran in a crazy zig-zag or on top of a building when in fact you ran in a straight line on a sidewalk, for example)

I always wait for the green GPS ring, then I wait for the pace to settle down to 0:00, before starting an activity. (Unless I’m in a huge hurry.) Only downside is it can take anywhere from a few seconds to a couple of minutes to get a “real” GPS lock this way.

This is also why it’s def worth opening your run activity several minutes before the start of a race, so you can get a solid GPS lock and avoid this kind of problem.