I often use my FR60 or 310xt for indoor spin classes and still label the workout "Cycling".
Recently I decided it may be interesting to see my total miles since Jan 1, so I did a quick report on Garmin Connect. I could see the mileage I've put on my 'real bike' so far this year, but the average speed appears to include all of the 0 mph data from spin classes.
Essentially it looks like it adds the time for all "cycling" activities and the total distance and calculates it, rather than doing a more complicated calculation only including workouts with some distance / speed themselves.
This just makes the average useless unless I re-categorize my spin sessions to help with report generation...