The real question is how you calculate your average. I can do 1 speed for a 30 minute sprint and another speed entirely on a 2 hour ride thru the hills. There is also the problem that the computer only calculates the average while the wheel is spinning.
If I leave my house at 8 am, stop twice for refreshments and get home at 11, my computer says I averaged 16 mph for 40 miles. According to my math, I averaged 13.3.