I knew I'd get challenged on that.

It is actually pretty common to see higher power averages outdoors if the terrain is pretty level (+/- less than 15 feet per mile with no challenging sections that cause a real bog-down) and the road is smooth. Not completely sure why that is, but some of it must be due to better cooling outdoors. Motivation outdoors vs. indoors is often cited and could be significant for some people, but that's not really measurable so it's hard to say how much difference it makes.

On hilly courses, though, average power will almost certainly be lower than on the trainer unless a steady wattage is adhered to without any regard for speed. Repeated efforts well above the average wattage (each followed by a recovery period) never proves as efficient as steady effort or negative splits. The hour record is never set by alternating 36 mph sections with 27 mph sections.