As Johnny J says if you consider going wide at corners, avoiding other runners, parked cars, pedestrians etc etc you would expect to actually run more than 10 miles in a 10 mile road race. I suspect you'll find your GPS device shows 1 - 2% above the race distance in other similar races as well. If it showed that you'd covered less than 10 miles instead then that would be a reason to worry!
I agree with Porcus, we all ran the same route - in a very well organised and enjoyable event - so what does it matter?